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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
Sons of Horus and Imperial Fists battle on Cthonia
Conflict Horus Heresy
Date 006-014.M31[1][2]
Location Cthonia
Outcome Imperial victory
Loyalists Traitors
Lord Castellan Evander Garrius
Lieutenant Annael (Final Phase)
Captain Vheren Ashurhaddon (Relief Force)
Chieftain Kovarny (KIA)
Dark Apostle Ul-Buus
See order of battle See order of battle
Imperial Fists almost entirely destroyed All Destroyed

The Siege of Cthonia began in 006.M31 during the Horus Heresy, as several Sons of Horus splinter fleets attempted to break the Imperial Fists' blockade of their Homeworld, Cthonia.[1][2]


Cthonia been blockaded by the Imperial Fists since the Heresy in 006.M31,[2] as given the situation elsewhere in the Galaxy the world was too powerful to fully conquer. To do so, would have required the Imperium to commit substantial strength, that was needed to fight Warmaster Horus' forces elsewhere. However, as the Warmaster approached the Sol System,[1] several Sons of Horus splinter fleets attempted to break the blockade, most notably a relief force (dubbed the "True Sons of Cthonia") under Captain Vheren Ashurhaddon, who destroyed many loyalist ships in orbit before reinforcing the planet with a large fleet that outnumbered the loyalists 3-to-1.[3e][3b]

The Imperial Fists under Lord Castellan Evander Garrius launched a desperate counterattack to prevent the freeing of Cthonia. If they did so, then the Warmaster's assault on Terra, would be reinforced with thousands of new Sons of Horus Legionaries.[1][3a] As the conflict dragged on, the Imperial Fists besiegers began to recruit new Space Marines from Cthonia, resulting in the ironic situation that by the later stages of the siege many Imperial Fists were natives whereas the Sons of Horus relief troops had been recruited from other planets and were foreigners.[3a]

The siege's main phase began in 012.M31,[1] and became focused around the Traitor's Gate (formerly known as Lupercal's Gate) Hive of the world. 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 several years of bloody stalemate, a Dreadwing force of the Dark Angels arrived shortly after the end of the Siege of Terra. By then, the Imperial Fists positions on Cthonia seemed to have been heavily reduced, but the Dark Angels commander Annael decided to quickly set off Vortex Bombs on Cthonia rather than waste the element of surprise to warn them to evacuate. The Dreadwing launched a shock assault on Cthonia in order to set the Vortex Bomb, but 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. The Dark Angels and Fists both took heavy casualties, but some forces managed to evacuate as the vortex bomb detonated and shattered Cthonia in half.[3d]

Order of battle




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 Cthonia
007-008.M31 Battle of PhallBattle of RavendelveRuin of MaedranBattle 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 NocturneBattle of PlutoSiege of InwitBurning of Ohmn-Mat
011-014.M31 Lorin Alpha CampaignSubjugation of TyrinthMalagant ConflictBattle of the Kalium GateBattle of CatallusAxandrian IncidentBattle 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-GarmonDefence of RyzaBattle of ThagriaPassage of AngelsThassos IncidentBattle of Zhao-ArkhadSerpent's CoilSiege of BarbarusBattle of VezdellBurning of VrexorDawn of DesolationDeath of ChemosBattle of Luth TyreYdursk IncidentSolar WarRaid on LunaSiege of TerraGreat Scouring