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Siege of Cthonia

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Targetdrone.gif This article is about the Horus Heresy battle; for the Rulebook, see Campaigns of the Age of Darkness: The Siege of Cthonia.
Siege of Cthonia
CRArt.jpg
Sons of Horus and Imperial Fists battle on Cthonia
Conflict Horus Heresy
Date 006.M31 (First Siege)
006-013.M31 (guerrilla campaign)
013-014.M31 (Second Siege)
Location Cthonia
Outcome Imperial victory
Combatants
Loyalists (Imperial Fists-led faction) Traitors Loyalists (Dark Angels)
Commanders
Lord Castellan Evander Garrius[4c]
Consul-Captain Imran Adarus[4f]
Archmagos Arkai Nomus[4c]
Captain Syban Nethkal (KIA)[4c]
Magister Sul Kontep (KIA)[4c]
Vigil-Commander Ambriel Malineux[4c]
Lord Marshal Sember Atriox[4c]
Captain Vheren Ashurhaddon[4d]
Captain Zedak Mordakar (KIA)[4d]
Archmagos Draykavac[4d]
Archmagos Ferrok Trahn[4d]
"Incursus"[4e]
Chieftain Ithan Ryaeve[4f]
Chieftain Kovarny (KIA)
Centurion Boros Kurn[4g]
Dark Apostle Ul-Buus (KIA)
Eskaton Marduk Sedras[4k]
Lieutenant Annael[3d]
Strength
See order of battle See order of battle See order of battle
Losses/Survivors
Mostly destroyed Mostly destroyed Moderate

The Siege of Cthonia was a three-stage battle of the Horus Heresy. In the so-called "First Siege", Cthonia was largely conquered by an Imperial Fists-led force in 006.M31. However, Sons of Horus guerrillas remained active in the planet's vast underground caverns, waging a guerrilla campaign over the next years.[4b] After this period of contested Imperial rule, a large Sons of Horus-led splinter fleet launched its own invasion to retake their Homeworld in 013.M31, starting the "Second Siege".[4e] This conflict culminated in the Dark Angels destroying Cthonia in 014.M31, killing most of the local traitors as well as loyalists.[4l]

History

First Siege

When Horus rose in rebellion in 005.M31, his Legion's Homeworld Cthonia was left rather isolated within Imperial space. Even though the Sons of Horus Primarch had prepared his rebellion for years, he had invested little energy in reinforcing or fortifying Cthonia. Evidently, he regarded the planet as unimportant to his wider military efforts. Accordingly, only two Sons of Horus chapters and one Word Bearers company were stationed on the planet, left alone to face the loyalists wrath.[4b]

Despite Cthonia's small garrison, the Imperium could not afford to leave Cthonia be, as it could possibly act as a Traitor forward base for strikes into the nearby Sol System. Imperial Fists Primarch and loyalist chief defensive commander Rogal Dorn consequently assembled a strong force to capture Horus' homeworld, consisting of the Imperial Fists' 9th and 14th Battalions, the Space Wolves 13th Great Company, three Adeptus Custodes sodalities,[4b] and Mechanicum Taghmata.[4a] Led by Lord Castellan Evander Garrius, this contingent invaded in 006.M31, beginning the First Siege of Cthonia.[4b]

Vastly outmatched, the Traitors did not contest the invasion force in the void around Cthonia, allowing the Imperials to land unopposed. Instead, the Traitor garrison focused on defending Cthonia's main settlement, the Hive Lupercal's Gate, as well as the Terminal Atlas. First blood was spilled when the Loyalist vanguard, consisting of the 13th Great Company overran the Sons of Horus defending the outer areas of Lupercal's Gate. They were followed by the Imperial Fists who advanced into the vast spire of the capital settlement. Spearheaded by Garrius and his 456th Company, the Loyalists broke through the main gateway and eventually into the former throne room of Primarch Horus. However, this attack was carried out with unwarranted speed due to Garrius' eagerness to humble his former allies, costing the Imperial Fists much more troops as would have been neccessary. The Sons of Horus, meanwhile, already fought a guerrilla campaign, using the dense hive city to cover their counter-attacks as well their gradual retreat.[4b]

Eventually, the Loyalists realized that the Traitors were not focused on holding Lupercal's Gate, but sabotaging key systems to render the planet's industry, infrastructure, and orbital defenses useless for the Imperium. Though Garrius' force hurried to stop the saboteurs, the Traitors successfully crippled many systems and eventually retreated into Cthonia's underground caverns from where they could continue their resistance. Overall, the First Siege was a rather hollow victory for the Imperium, and Dorn was upset at the unneccessary losses and failures of this battle.[4b] A number of Sons of Horus commanders were captured in the First Siege. They were subsequently brought to Terra and publicly executed.[4a]

Map of the Siege of Cthonia[4e]

Cthonia under Imperial control

After conquering Cthonia, Rogal Dorn ordered the entire planet to be reformed according to Loyalist doctrine. This included renaming Lupercal's Gate to "Traitor's Gate" and removing all traces of the Sons of Horus from the planet, stripping its resources for the war effort, devoting its population to work in factories to provide Imperial materiel, and mass recruiting locals into the ranks of the Imperial Fists. This task was given to Garrius who was appointed the planet's de facto ruler by Dorn; most understood this to be censure and punishment for his previous failures.[4b]

Before long, the Imperium also relocated most of the invasion force from Cthonia to other war zones. This left Garrius with a small garrison, centered around his 456th Company. In addition, he was provided some Imperial Army units and some of the new Imperial Fists recruits from Cthonia's population (though most would be sent off-world).[4b] The occupation was further supported by the Graian Mechanicum under Archmagos Arkai Nomus who used Cthonia to repair damaged Titans.[4c][4e] Despite his unfavorable position, Garrius set about transforming Cthonia into a loyalist stronghold. He ordered the relocation of the planet's population from its caverns into the surface settlements, leaving the underground to the traitor guerrillas. He then began rebuilding the planet's industry and Space Marine recruitment process, largely succeeding in this despite constant guerrilla attacks.[4b]

Despite their efforts to purge the remaining traitor presence on Cthonia, however, the loyalists remained largely ignorant that the remnants of the initial Sons of Horus garrison were reorganizing and rebuilding their strength. Deep inside Cthonia's caverns, at a underground bastion nicknamed "Tyrant's Den" close to the planet's cold core, the traitors rallied under Centurion Boros Kurn. Though they were too weak in numbers and equipment to pose a serious threat to Garrius, Kurn ordered his forces to mass recruit locals as ad hoc Inductii Space Marines, build secret underground bases, and scout the loyalist positions.[4g] Over time, the Sons of Horus guerrillas grew to several thousand members (albeit many instable and mutated Inductii), waiting for an opportunity to remerge from the darkness.[4g][4j]

In 007.M31, as the Ruinstorm was first created, Cthonia became cut off from the rest of Segmentum Solar for a limited time. The Sons of Horus guerrillas, supported by Word Bearers elements, exploited the following unrest by launching a major attack on the Imperials from their caverns. The assault was repulsed by Garrius' forces under great cost.[4a] Garrius subsequently requested reinforcements from Dorn. The Primarch could only spare few troops, and thus sent loyalist Fifth Fellowship of the Thousand Sons to join the garrison. This contingent led by Sul Kontep were survivors of the Burning of Prospero who had thrown themselves at the Emperor's mercy. They had been offered a chance at redemption by serving the Imperium in the war against Horus,[4b] though they were overseen by a cadre of Sisters of Silence led by Ambriel Malineux.[4b][4c] Garrius trusted these twice-traitors little, stationed them at Pharos Obscurus, a lumen tower far away from the other Imperials. From there, the Thousand Sons were ordered to hunt for Traitors in the underground caverns.[4b]

Over the next years, Garrius' garrison was reinforced by Loyalist survivors from the battles of Tallarn and Beta-Garmon, as well as various Shattered Legion warbands. These used Cthonia to strike into Traitor-held space in the galactic south.[4b] However, one group of alleged Salamanders (arriving in 011.M31) was soon revealed to be an Alpha Legion team in disguise. Though the Imperials drove of the Alpha Legion agents, the latter succeeded in delivering supplies and other reinforcements of the traitor guerrillas under the surface. From 012.M31, a growing number of scattered loyalist warbands arrived at Cthonia, fleeing the Passage of the Angels.[4a] As the conflict dragged on, the Imperial Fists recruited more and more Space Marines from Cthonia, resulting in the ironic situation that by the Second Siege at least a third of the Imperial Fists garrison were natives whereas the Sons of Horus relief troops had been largely recruited from other planets and were foreigners.[3a][4c]

Second Siege

Ashurhaddon assembles his force

Map of the subterranean battles of the Siege of Cthonia[4f]

The Second Siege of Cthonia resulted from divisions within the Sons of Horus Legion. A large section of the Legion, led by Captain Vheren Ashurhaddon, had begun to resent the growing corruption in their midst. This faction believed the Sons of Horus to be on a path to ruination, seeing possible salvation in re-embracing the old ways and honor of their homeworld Cthonia. As their warnings and calls for reform were ignored by Horus and his inner circle, Ashurhaddon's followers organized into a warrior society dubbed the "True Sons of Cthonia". This society grew increasingly large, attracting not just veterans longing for days gone, but also newly recruited Inductii who hoped they could find an identity in the traditions of a homeworld they had never seen. Either way, the "True Sons" considered the Imperial occupation as a great dishonor to the Legion and wanted to liberate it.[4e]

The opportunity for Ashurhaddon's faction arrived in 013.M31, as the Solar War came to an end and Horus began to plan an invasion of Terra itself. At this point, the Primarch finally allowed the "True Sons" to attempt a reconquest of Cthonia. Later scholars assumed Horus had sanctioned the attack as he could spare the strength of the "True Sons" and thought the retaking of his homeworld might be a potent symbolic victory; he was leading such a mighty host at this point, while the loyalists in the Sol System had already been severely weakened, that Ashurhaddon's troops would not be missed at Terra. Reportedly, only Alpharius warned Horus that the operation was strategically unsound, but the Alpha Legion Primarch's words were ignored.[4e]

Thus, the "True Sons" began to assemble in the Beta-Garmon System. At this point, they numbered about 20,000 Space Marines and a significant fleet, led by Ashurhaddon's flagship Tomb of Gold. Despite their eagerness to start their offensive, however, the "True Sons" took care and time for preparations, placing great value on pragmatism due to their devotion to Cthonian tradition. Accordingly, Ashurhaddon contacted various other traitor factions which might join their campaign, such as various Imperial Army units of the Cthonian Headhunters and a Dark Mechanicum force loyal to the Forge World Voss. The latter had long held the rights to extract Cthonia's resources and was thus willing to support the "True Sons", finding further value in the chance to humble the forces of Graia that were backing Garrius' garrison.[4e]

The "True Sons" were also joined by more enigmatic groups such as a strong Word Bearers force led by Zedak Mordakar of the Flayed Hand and an Alpha Legion unit. Many "True Sons" initially argued against allowing the Word Bearers join their campaign, seeing them as the origin of their own Legion's corruption. However, Mordakar and his men appeared cordial and diplomatic, and Ashurhaddon reminded his followers that the Cthonian way was to judge a warrior by their actions.[4e] Thus, the Word Bearers (counting thousands of Space Marines) were included in his force.[4c][4e] Ultimately, Ashurhaddon's assumption on the honor of Mordakar's contingent proved to be naive; the Word Bearers were secretly acting on Erebus' orders and planning to bring doom to Cthonia.[4e][4j] Last of all, the Alpha Legion contingent on scout cruiser Sigma 9-14 just showed up and linked up with the "True Sons" fleet. This group, led by a warrior only known as "Incursus", ignored most attempts at contact; Ashurhaddon ultimately tolerated the Alpha Legion presence, deeming it too much trouble to remove them from his invasion force.[4e]

Void battle and first landing

The "True Sons" invasion force initially allowed the traitor main fleet of Horus, heading to begin the Siege of Terra, to enter the Warp first. Afterward, Ashurhaddon's forces began their own journey at about 759013.M31. With the aid of Mordakar's Word Bearers, the "True Sons" managed to cross the Warp rather quickly, though their fleet's cohesion suffered and consequently entered the Cthonia System piecemeal. The first traitor ship to arrive was the Sigma 9-14 which was immediately attacked by two loyalist patrol light frigates, the Blade of Midnight and Shadow-eye of the garrison's Raven Guard contingent. The Alpha Legion ship masterfully avoided the destruction for four minutes, when the rest of Ashurhaddon's advance elements began arriving. Confronted with an entire fleet, the Shadow-eye sacrificed itself to hold the traitors long enough so that the Blade of Midnight could escape.[4e]

After this initial clash, the main void battle began. Once assembled in full, the traitor fleet outnumbered the loyalist fleet defending Cthonia 3-to-1:[3b] Ashurhaddon commanded 5 capital ships, 50 ships of the line, and over 300 destroyers and corvettes, whereas Garrius' naval contingent included fewer than 100 destroyers and picket ships, 23 ships of ther line, and three capital ships (namely the Imperial Fists Gloriana Class Battleship Burden of Duty, the Iron Hands Golgotha Class Factory Ship Grendel, and the Fortuna Class Trade-Barque Warp-spite). Knowning his force was outmatched but forewarned by the Blade of Midnight, Garrius resolved to size some advantage by striking first. Accordingly, he ordered his fleet to go on the offensive. Spearheaded by the Grendel, whose crew of fanatical Iron Hands had vowed to die rather than cede ground to the traitors, the loyalist fleet broke into the traitor fleet which was still in disorder after exiting the Warp. This furious assault momentarily threatened to inflict heavy losses on the traitors, especially as the Grendel was close to fire up the Dark Mechanicum transport ships carrying Ashurhaddon's Titan contingent. However, the Imperials were stopped by the unexpected arrival of a new force; a fleet loyal to Cyclothrathe led by Archmagos Draykavac left the Warp and fell upon the loyalists, crippling the Grendel. Further reinforced, the traitors crushed the loyalist fleet, with only about a dozen Imperial cruisers breaking through to escape into deep space,[4e] eventually hiding in the system's asteriod fields.[4k]

Once the traitors arrived in Cthonia's orbit, however, they discovered that Garrius had ordered the crippling of the planet's orbital plate, thus forcing the invaders to attempt a direct landing onto the heavily defended surface instead of using the more convenient orbital ladder.[4e] This caused heavy disputes in the leading council of the traitors. Ashurhaddon argued for a well-prepared landing in force further away from Cthonia's population centers and the loyalists' guns. However, a group of more hot-headed officers, most importantly Chieftain Ithan Ryaeve, clamoured for an immediate assault - this group mainly consisted of Inductii who felt they had to prove their worth by quickly overcoming Garrius' defenses and thus restore Cthonia's honor. The discussion was decided by Mordakar of the Word Bearers who sided with Ryaeve, but also offered that his troops could lead the attack if the Sons of Horus were wishing to avoid heavy losses. This suggestion implied that his Legion was cowardly, forcing Ashurhaddon to sanction an immediate attack to maintain the honor of the "True Sons".[4f] Unbeknownst to him, this was the first part of Mordakar's plan - to spill as much blood as possible (regardless of origin) at certain areas of Cthonia for an upcoming Chaos ritual.[4k]

Either way, the "True Sons" now had to commit their forces. To make the best out of the situation, Ashurhaddon opted for a two-stage attack between two loyalist bastions, namely Terminal Atlas and Pharos Obscurus. This area was covered by the Imperial Fists' heavy orbital guns, but these weapons would not be able to stop a mass landing by smaller targets. Accordingly, the traitor commander would first deploy Ryaeve's 68th Chapter via Drop Pods into this zone; after these troops had secured their landing area, the veteran 5th Chapter would follow in Thunderhawks to extend the traitor-held territory and strike at the nearby Praesidium Arx, a stronghold where Garrius had put his command post. If everything went according to plan, the traitors would probably suffered heavy losses, but also cripple the entire loyalist defensive network in one fell swoop.[4f]

However, the operation derailed once the first wave under Ryaeve had broken through the air defenses and was landing on the ground. As the Drop Pods neared the ground, they set off powerful grav-mines, inflicting catastrophic losses on the 68th Chapter. In truth, Garrius had set up a trap in the area, expecting a traitor landing there and setting up a killing field. Even as Ryaeve's men were butchered by the mines, Raven Guard Deliverers were teleported into their midst to inflict further damage. Even though the 68th Chapter's situation appeared hopeless, Ryaeve managed to rally his troops and organized localized counter-attacks on the Deliverers. For a moment, it appeared as if his force could overcome the loyalists and break out of the minefield, only for the Raven Guard's second wave to arrive - and among these was Kaedes Nex, an "infamous assain" who shot Ryaeve and thereby threw the 68th Chapter back into confusion. The initial landing party was only saved by the arrival of the 5th Chapter which was able to evacuate the 68th Chapter's survivors, including a badly wounded Ryaeve.[4f]

Second landing and battle of Shaitan's Bowl

The second landing of the "True Sons", this time following Ashurhaddon's original plan, went much better than the first and allowed the traitors to entrench themselves on Cthonia's surface. This time, the "True Sons" conducted an organized mass deployment in the wastelands west of the Vorhek mountains; shielded by his navy and largely outside the reach of Garrius' guns, the first waves of Ashurhaddon's force were able to descend in good order. The Imperial leadership was divided as to how to react to the second landing; Lord-Commander Sember Atriox of the Imperial Army and Archmagos Nomus pleaded for an immediate counter-attack, whereas Captain Syban Nethkal of the Raven Guard and Consul-Captain Imran Adarus of the Imperial Fists advocated a defensive approach. Garrius took the side of the aggressive officers, and the counter-attack went ahead.[4f] Garrius believed that the fall of Cthonia would allow for the Warmaster's assault on Terra to be reinforced with thousands of new Sons of Horus Legionaries, and thus refused to give any ground to the invaders.[1][3a]

The loyalists' first wave of the counter-attack consisted of a large numbers of aircraft, Land Speeders, Sabre Strike Tanks, and mechanised infantry. They were followed by a slower second wave, consisting of heavier tanks as well as Imperial Knights and Titans. The first wave arrived at the traitor landing zone within half a day, and indeed disrupted the traitors, though suffered heavy losses at the hands of Ashurhaddon's own aircraft contingents. By noon, the traitors had succeeded in deploying 500,000n infantry and about 1,000 armoured vehicles, though the traitors had not yet been able to land their own titans. Within the following night, Garrius' second wave approached the traitors, forcing Ashurhaddon to hastily mobilize a defense in the region before the landing area, lest the Imperials break into his newly constructed camp. The following battle erupted at a salt-flat called "Shaitan's Bowl".[4f]

The engagement at Shaitan's Bowl began unusually with Consul-Captain Adarus, placed in command of the counter-attack, broadcasting a message to the traitors. In it, he declared that he had once fought alongside the Sons of Horus' 19th Chapter, knowning them as honorable warriors, and stated that "in memory of old oaths I offer one last salute before I must destroy them". In response, the 19th Chapter's lead tank raised its own gun in silent greeting. For a moment, there was silence between the armies, before someone - possibly Mordakar's Word Bearers who had identified the grounds as yet another ritual site - opened fire. The resulting battle was catastrophic and furious, as if "reminded once more of the betrayal at the heart of the Horus Heresy, [...] both sides had gone mad and sought only death". Fighting lasted into the next day, when the first Loyalist Titans of Legio Astraman entered the fight. At first, the Loyalist Titans pushed back the traitors who only stopped their advance by sending Ordinatus engines which were soon overwhelmed as well. However, the Ordinatus engines had bought enough time for the newly landed Titans of Legio Ulricon, Legio Andrastus, and Legio Magna to fully deploy and respond to the loyalist counter-attack. Now, the Legio Astraman contingent was outnumbered 2-to-1 by Traitor Titans, and the battle of Shaitan's Bowl was decided. As it became clear that the loyalists were outgunned, they retreated, but were threatened to be overwhelmed. In response, the Legio Astraman Titans sacrificed themselves as a rearguard to allow the remaining loyalists to leave the battlefield. Overall, both sides had suffered terribly at Shaitan's Bowl, but the Imperials had failed in reducing the traitors' landing zone as well as lost most of their Titans.[4f]

Raiding war and butchery at Bastion Aleph-prime

The defeat at Shaitan's Bowl threw the loyalists into disarray, as not all traitors had confronted Garrius' offensive. The remanants of the 5th and 68th Chapters under Chieftain Ryaeve had relocated to the wastes beyond the Atrakaan Peninsula to reorganize after their heavy losses during the intial landing attempt. As Adarus had led the majority of the loyalist garrison on the offensive, Ryaeve's two thousand Space Marines had exploited this to fly their Thunderhawks alongside the coast to outflank and surprise the weakened loyalist defenses. The Sons of Horus thus fell upon the small garrison of the strategically important Terminal Atlas, consisting of 100 Imperial Fists and a cohort of Necromundan Solar Auxilia. Though the loyalists put up a dogged defense, they were decimated by the traitors; yet, before they could be overwhelmed, Terminal Atlas' defenders were reinforced by a contingent of Sul Kontep's Thousand Sons. Combined, the garrison and Kontep's troops held of the Sons of Horus long enough for further Imperial Fists to arrive, forcing Ryaeve to retreat.[4g]

Despite the successful defense of Terminal Atlas, Garrius was furious that his fortifications had been outflanked at all and angrily ordered all his defenses to be further strengthened. Meanwhile, the Thousand Sons had gained new respect among most loyalist defenders of Cthonia, even if Garrius continued to mistrust them. Either way, the war for the planet subsequently entered a stalemate, as both sides were exhausted by their heavy losses in the last clashes. Ashurhaddon limited himself to probing attacks organized by his mobile forces and Alpha Legion allies, while reorganizing his main force and attempting to find a way to overcome the heavy Imperial defenses. In particular, the Imperial Fists had built "The Wall", a series of connected bunkers, outposts, and fortresses that shielded the Praesidium Arx and Traitor's Gate Hive. Garrius countered the traitor raids by deploying Nethkal's adaptable Raven Guard and the remaining Iron Hands armoured contingents. At the same time, the Thousand Sons and Sisters of Silence were ordered to continue their underground patrols.[4g]

The next phase of the siege was thus marked by a raiding war aboveground, while both sides prepared for a major clash at The Wall. To some degree, this raiding conflict devolved into a private war between the Alpha Legion and Raven Guard as the two sides tried to outdo each other in guerrilla warfare, ambushes, and strikes at logistical targets. In particular, Alpha egion leader "Incursus" and Raven Guard assassin Kaedes Nex began to outright hunt for each other, clashing during murderous skirmishes at outpost Phi-19 and the "Serpent Jaws" with neither side gaining the upper hand. The outriders and tanks of the Imperial Fists' 19th Company and Sons of Horus' 5th Chapter also repeatedly battled for the outlying outposts of The Wall.[4g]

Meanhile, Ashurhaddon's forces had made contact with the remnants of Cthonia's original garrison under Boros Kurn, and began to coordinate with them to turn the underground war in their favor. In particular, Archmagos Draykavac sent hordes of twisted Vorax Class Robot into the caves under the Yedig Expanse; these machines succeeded in driven back the underground Loyalist garrisons, but also butchered thousands of Cthonian civilians. Though Ashurhaddon was disturbed by this butchery, he assumed that it was neccessary to further his homeworld's reconquest. Either way, the underground offensive and the Alpha Legion raids revealed that The Wall may had a vulnerable section in its center; Ashurhaddon moved to exploit this by coordinating a quick assault on six Imperial outposts (Phi-19, 21, 36 and Theta-03, 07, 09) to eliminate the Loyalists' forward reconnaissance. Afterward, he sent a major armoured contingent backed by Dark Mechanicum troops to strike the The Wall's central fortress, Bastion Aleph-prime.[4g] Unbeknownst to most traitors, this location was yet another seal in Mordakar's occult nexus; thus, the Word Bearers eagerly assisted in this attack.[4g][4h] The Traitors seemingly succeeded in surprising the fortress, manned by just 500 Imperial Fists and Charonid Sentinels under Consul-Captain Adarus, and furiously assaulted it. The Traitors's first wave, mainly consisting of robots and possessed Word Bearers, suffered heavy losses in their attempt to breach the fortress despite having the moment of surprise, as the Imperial Fists had set up excellent defenses including massive minefields and phosphex bombs.[4g]

"See how well the sons of Cthonia fight in yellow, when you are dead and forgotten they will all make fine Imperial Fists."
"They will never be yours."
- Exchange between Evander Garrius and Vheren Ashurhaddon during their duel in Bastion Aleph-prime[4h]

Once the Imperial defenders' ammunition had been largely spent in breaking traitors' first wave, the Sons of Horus surged forward. Led by Chieftain Ryaeve, they managed to breach Aleph-prime's bunkers and pushed into its corridors, overwhelming the loyalists and forcing Adarus to order a retreat.[4g] The Wall's outer defense had been broken, and Ashurhaddon ordered a mass offensive to exploit the breakthrough. Even as he attempted to coordinate his troops, however, the Word Bearers and many Sons of Horus rushed forward in such a maddened frenzy that they lost any cohesion. This forward contingents fell upon Adarus' retreating troops on the plains behind Aleph-prime, only for the loyalists to trigger a series of pre-prepared mines. To Ashurhaddon's horror, the loyalists had prepred a large ambush, and even as the mines bloodied the traitors' forward contingents, hidden tunnels opened. From these, large numbers of Imperial Fists emerged and fell upon the traitors. Even Ashurhaddon himself came under attack, as he had set up a makeshift headquarters in the ruins of Aleph-prime, only for loyalists under Garrius to emerge from vaults beneath the bastion. The two chief commanders even engaged in a private duel.[4h]

Complete disaster was avoided by the traitors thanks to Ashurhaddon's planning. Though the traitor commander had not expected an ambush or deception of this scale, he had assumed that the Imperials might have some hidden defenses in place. Thus, he had held strong forces including Mordakar's personal troops in reserve; these contingents now moved forward to rescue their comrades. However, the forces under Mordakar's personal leadership advanced slowly; they would not have arrived in time to save Ashurhaddon himself. The traitor leader was instead rescued by Dark Mechanicum Thallax under Draykavac. As Garrius was forced to fall back, Ashurhaddon ordered all traitors still on the plains behind Aleph-prime to retreat. In contrast to the earlier chaotic offensive, this retreat was carried out methodically, with Ryaeve skillfully commanding a rearguard composed of Sons of Horus and Cthonian Headhunters Solar Auxilia. Seeing little chance to escape, Ryaeve and his comrades organized a last stand, impressing even the Imperial Fists with their determination. Adarus, remembering once more the long-gone greatness of the Luna Wolves in this moment, stepped forward and challenged Ryaeve to a duel. Fighting stopped to allow for this clash to take place; yet the duel was anticlimactic, with Adarus easily parrying Ryaeve's hate-filled attacks and heavily wounding him with just one swordstroke. Despite his victory, however, the Imperial Fists Consul-Captain spared the chieftain and his few remaining followers, merely rebuking Ryaeve to finally live up to the legacy of the Luna Wolves he once called his oath-brothers.[4h]

After this point, both traitors as well as loyalists retreated from the battlefield. Garrius had never intended to hold the outer sections of The Wall, instead planning a major ambush from the start. Accordingly, he was furious that any traitors had escaped the slaughter on the plains, punishing Adarus for allowing the traitor rearguard to retreat in honor. Garrius subsequently organized his forces along The Wall's inner section, while the traitors rallied their forces and planned their next moves.[4h]

War for the core territories

The siege's main phase ultimately became focused around the Traitor's Gate Hive. Cthonia's vast subterranean networks and gang culture made the fighting grueling and brutal, with repeated attacks by both sides often achieving little. The Imperial Fists found themselves trapped between the Sons of Horus both above and below, causing their advance to falter.[3a] Besides their Astartes warriors, the Sons of Horus employed unaugmented Human chain gangs as well as newly inducted hastily produced Initiates as cannon fodder.[3c] To try and break the stalemate, Ashurhaddon used Chieftain Kovarny's Heathens Reaver Attack Squad to kill Garrius, who was located within the Traitor's Gate. His death would then topple the Imperium's resistance and allow the Sons of Horus to finally advance their invasion into the Hive. However due to fighting together during the Great Crusade, Kovarny considered Garrius to be his Brother and instead secretly sought to save the Imperial Fist by converting him to the Traitor's cause. Garrius would refuse to do so, though, and ultimately killed Kovarny before his remaining forces escaped from the Chieftain's pursuing Reaver Squad.[3e]

After months of heavy fighting, Garrius' force was hard-pressed and close to being overrun by the traitors. However, he and his troops still clung to a number of bastions and were engaged in counter-attacks. In fact, having become aware that the Word Bearers were attempting some kind of ritual near the planet's core, Sul Kontep's loyalist Thousand Sons and Vigil-Commander Malineux's Sisters of Silence launched an underground offensive. Their combined force first encountered what remained of Boros Kurn's troops. These former Sons of Horus Inductii had been horribly mutated by the Word Bearers under Mordakar, turning them into mindless monsters that were now flooding upwards to join the battle on the surface. Fighting through these hordes, Kontep realized that the Word Bearers' ritual was designed to somehow influence the Siege of Terra. With the sanction of the Sisters of Silence, the Thousand Sons proceeded to unleash their full psychic might to break through the traitors and stop the ritual. Albeit they suffered massive losses, Kotep and his followers suceeded, killing Mordakar and shattering his ritual site. This, however, opened a Warp Rift, causing hordes of Daemons to pour into the caverns of Cthonia. While the surviving Thousand Sons and Sisters of Silence were able to retreat upwards, the Daemons butchered all remaining Sons of Horus in the Tyrant's Den before also joining the battle on the surface.[4j][4k]

As they fell upon Cthonia's surface, the Inductii mutants and Daemons first broke through at loyalist-dominated positions, meaning that Garrius' remaining force -most importantly at Traitor's Gate- felt the brunt of their wrath. As Ashurhaddon's troops advanced, however, hoping to exploit the anarchy to their advantage, the Chaos creatures also attacked them.[4j][4k] As a result, almost all order broke down in the pandemonium of the multi-sided fighting. Many veterans on both sides, including Garrius and Ashurhaddon, refused to budge even under these conditions. They still focused on killing each other and achieving by-then meaningless strategic objectives instead of addressing the threat posed by the Daemonic hordes. Ashurhaddon led his main force to breach Traitor's Gate, capturing his Primarch's throne room, only for Garrius and his elite Huscarls to follow to retake this old seat of power. Not all were so stubborn, however; some loyalists and traitors, facing the horros of the Warp, opted to unite their strength against a common foe instead of perishing in fratricidal violence.[4k][4l]

Just as Cthonia fell into madness, a new force entered the battle: A large Dark Angels fleet arrived in the system, just after the end of the Siege of Terra.[3d][4lk] This force was led by Eskaton Marduk Sedras and mainly consisted of Dreadwing elements; it had come, allegedly on the orders of Lion El'Jonson, to bring destruction to all traitor worlds, and this included Cthonia. The Dark Angels easily overwhelmed the traitor fleet in Cthonia's orbit, though allowed its defeated remnants to flee. This opportunity was exploited by the remaining fleet of Garrius' garrison which returned to fall upn the retreating traitor ships. The Dark Angels ignored this space battle, instead broadcasting a message to those fighting on Cthonia itself.[4k]

"We are judgement come at last. We offer one chance only, cease all conflict and leave the planet. Stand against us and die or flee and bear witness to the rewards of treachery. We are the death of worlds and this is the Eskaton."
- Broadcast by Eskaton Marduk Sedras of the Dark Angels Dreadwing[4k]

The Dark Angels ordered all on the planet to either flee or die; this would be their first and final ultimatum. Unsurprisingly, the infighting forces on Cthonia neither could nor would heed this order. Ashurhaddon was unwilling to surrender his homeworld, Garrius would not abandon the planet entrusted to him by Rogal Dorn himself, and most others -combatants and civilians alike- were simply trapped amid the carnage with no way out. Unheeding of loyalist calls for aid, the Dark Angels then began their invasion. On their way down, the new arrivals further clarified their position by indiscriminately shooting down any aircraft within reach, both those of Sons of Horus as well as Raven Guard. The main Dark Angels force made planetfall at the landing zone of the True Sons, where they killed everyone, be they Space Marines, Cthonian Headhunters, wounded, or slave crews. Afterward, the Dreadwing destroyed the local traitor shuttles.[4k]

The Sons of Horus battle across Cthonia[42m]

Then, the Dark Angels turned to capture the Terminal Atlas. To do so, they first turned their capital ship's guns on the battlefield in front of it. There, traitors and loyalists were still locked in fierce combat. Uncaring, the Dark Angels launched an orbital bombardment that devastated both forces, notably destroying the Titans of both Traitor Legios as well as the loyalist Legio Osedax. Furious at this betrayal, Garrius ordered his orbital defenses to target the Dark Angels fleet, but most had been crippled during the siege and the remainder could do little. Their path cleared, the Dark Angels invasion force pushed forward, entering the Terimal Atlas. There, fighting still raged between the followers of Ashurhaddon and Garrius, allowing the Dark Angels to easily break through. Led by Eskaton Serdas himself, Deathwing Marines massacred the local Sons of Horus and Dark Mechanicum troops before forcing the Imperial Fists to retreat, "killing them only where they must". Just as they were securing Terminal Atlas, however, Serdas' men found themselves beset by the Daemonic hordes.[4k] They furiously fought to keep the area secure as they set up planet-destroying weaponry. Attempts by loyalist Imperial Army and Imperial Fists to seek aid from Serdas' force were answered with death, as the Dark Angels defended Terimal Atlas from both Daemons and loyalists alike.[4l]

Finally, Dark Angels commander Annael decided to quickly set off Vortex Bombs on Cthonia rather than give Garrius' troops one last warning to evacuate. However, Brother Karael disobeyed orders to warn the Fists to evacuate, causing a vicious Sons of Horus counterattack to fall on their position as a result.[3d] Whereas the Dark Angels retreated in largely good order, the surviving loyalists of Garrius' garrison attempted to flee as well as they could. Similarily, many traitors also realized that the planet was dying and sought to escape. In a few cases, the foes of previous months directly aided each other, with Consul-Captain Adarus of the Imperial Fists even giving his life to save Chieftain Ryaeve's troops from a Daemon ambush and thus allow them to escape Cthonia.[4l]

Order of battle of the First Siege

Imperial

Traitor

Order of battle of the Second Siege

Imperial garrison

Traitor

Dark Angels

Sources


Battles of the Horus Heresy
005-006.M31 Battle of Isstvan IIIFirst Battle of ProsperoWar Within the WebwayBattle of the Somnus CitadelUnrest on CalibanSchism of MarsBattle of DiamatDrop Site MassacreBattle of the Rangda SystemFirst Battle of ParamarManachean WarSignus CampaignTreachery at Advex-MorsSiege of CthoniaBattle for Felweather Keep‎
007-008.M31 Battle of PhallBattle of RavendelveRuin of MaerdanBattle of the Alaxxes NebulaPale Stars CampaignBreaking of the Perfect FortressChondax CampaignSecond Battle of ProsperoFirst Siege of Hydra CordatusBattle of the Furious AbyssBattle of CalthBattle of ArmaturaBetrayal at IthracaDeath of CanopusShadow CrusadeCrusade of IronDoom of BorminaDefence of TyrosDefence of the Three PlanetsBattle of UlixisAmbush at EspandorBattle of Aquila AtollBattle of Drooth IIPercepton CampaignBattle of IydrisThramas CrusadeFall of Baztel IIIBattle of VannaheimSecond Battle of ParamarBattle of Constanix IIMezoan CampaignBattle of BodtBattle of DwellErellian SubjugationSiege of Baal
009-010.M31 Battle of MolechBreaking of AnvillusXana IncursionCarnage of MoroxSangraal CampaignBattle of ArissakBattle of PerditusBattle of SothaDrussen AtrocityScouring of Gilden's StarBattle of NyrconBattle of TallarnCataclysm of IronBattle of NocturneWar of DrakesBattle of PlutoSiege of InwitBurning of Ohmn-MatBitter War
011-014.M31 Reaving of the Xibana ReachesLorin Alpha CampaignSubjugation of TyrinthMalagant ConflictBattle of the Kalium GateBattle of CatallusAxandrian IncidentBattle of the Haddon SystemJarrazr IncursionBattle of TralsakTarren SuppressionBalthor Sigma InterventionBattle of AbsolomScouring of the Ollanz ClusterBattle of ZepathSecond Battle of ZaramundBattle of AnuariBattle of PyrrhanSecond Battle of DavinArgolian MassacresBattle of TrisolianBattle of YarantBattle of KradeBattle of DelugeBattle of Heta-GladiusBattle of the Aragna ChainBattle of KallethBattle of the Diavanos SystemBattle of DesperationBattle of Beta-GarmonWar for AgaropsDefence of RyzaBattle of ThagriaPassage of AngelsThassos IncidentBattle of Zhao-ArkhadSerpent's CoilSiege of BarbarusForicaan CampaignBattle of VezdellBurning of VrexorDawn of DesolationDeath of ChemosBattle of Luth TyreYdursk IncidentFall of TenzebarSolar WarRaid on LunaSiege of TerraGreat Scouring