Hive Fleet Leviathan is the largest and greatest Tyranid Hive Fleet to invade the Galaxy. It attacked in 997.M41 from the galactic south. Unlike the majority of previous Hive Fleets that attacked from the Eastern Fringe, Leviathan instead skirted under the galactic rim of the galaxy and then pushed upwards through the galactic plane, spreading its forces across a broad front that covered the Ultima Segmentum, Segmentum Tempestus and even into the Segmentum Solar. Leviathan's strategy was to form its forces into two giant tendrils hundreds of light years apart in such a way that it stretched the Shadow in the Warp over the vast distance between the two tendrils, stopping all psychic contact between the trapped worlds and the Imperium and also blocking Imperial reinforcements from navigating the Warp towards the beleaguered worlds.
One of Leviathan's main tendrils, which was on a projected course directly to Terra, was eventually stopped at Tarsis Ultra. However, Leviathan as a whole remains undefeated and continues to assault the galaxy. Thus far Leviathan has consumed countless worlds. In response, the Imperium is reinforcing entire Systems, raising thousands of armies and dispatching dozens of Space Marine Chapters to troubled areas; meanwhile, several Craftworlds of Eldar are using ancient destructive weaponry to reduce planets to Dead Worlds, and the Tau are developing new technologies and weaponry to combat the Tyranids. However, Leviathan is only being slowed down rather than fully stopped, and it continues to assault all life in its path.
In late 997.M41 the Imperium first lost contact with several systems in the Segmentum Tempestus. Inquisitor Kryptman implemented a great astrotelepathic augury, which became known as the Kryptman Census, and eventually made contact with every documented planet on the southern fringe of the Imperium. The worlds that ceased to respond began to form a pattern, and Kryptman identified it as the approach of a hive fleet of terrifying size.
Leviathan's principle attack formed two tendrils hundreds of light years apart. The Imperium lost contact with the worlds trapped between these two "jaws", and if these jaws closed they would devour all trapped inside.
The planet of Tarsis Ultra stood directly in the path of one of Leviathan's main tendrils, the tendril on a course towards Terra. Tarsis Ultra was defended by the Ultramarines led by Captain Uriel Ventris, along with the aid the Mortifactors led by Chaplain Astador, and supported by the 10th Logres led by Colonel Octavius Rebalaq, the 933rd Death Korps of Krieg led by Colonel Trymon Stagler, and PDF Regiments headed by Major Satria. Inquisitor Kryptman was also present to impart his extensive knowledge of the Tyranid threat. When the Tyranids arrived in the system, in 999.M41 or in 509997.M41[6b] depending on the source, and consumed the planet Barbarus Prime, the Imperials launched an attack in space. Despite an initial victory in destroying a Hiveship, Kryptman wished to invoke Exterminatus upon the world Chordelis, as it was the next planet in the path of the Tyranids. Ventris was vehemently against this and supported an alternative solution to slow down the Tyranids by exploding a hydrogen-plasma refinery in space thereby destroying another Hiveship. However, Kryptman lied to Ventris and with the assistance of the Mortifactors invoked Exterminatus in spite of the Ultramarines' plan succeeding. The Imperials attempted the same space refinery trick again to destroy yet another Hiveship, but the Tyranids adapted to this ploy and the plan backfired on the Imperials, crippling their fleet and forcing the survivors to flee; Tarsis Ultra was left unprotected from space, allowing the Tyranids to launch their final planetary invasion.
Tarsis Ultra was besieged for several weeks, only just managing to hold out under the command of Uriel Ventris and later Kryptman. The tides eventually turned in Imperial favour when Ventris and a Deathwatch Kill-team captured a Lictor from the original wave of the invasion. Magos Biologis Vianco Locard reverse engineered the Lictor's genetic code to design a biological plague that would send an early generation Tyranid into hyper-evolution and thus kill the creature. If delivered to the heart of the swarm it could destroy the entire tendril. Ventris then joined the Deathwatch team in infiltrating the final remaining Hiveship and delivered the plague to the Norn-Queen. Slowly at first, but with gathering speed, the Tyranids turned upon and attacked each other. Their synaptic control had been lost. At this point, the remaining Space Marine contingent of both the Ultramarines and Mortifactors were down to sixteen men. The majority of the remaining Tyranids froze to death under the harsh conditions of Tarsis Ultra with a minority surviving and taking refuge in warmer surroundings. The Imperial forces launched a counter-offensive which destroyed the swarm and stopped the invasion. Leviathan's tendril, on a course to Terra, was dissipated and the once-doomed worlds caught in between Leviathan's two main tendrils began to register on Imperial augurs again.
The rest of Leviathan continued to rampage further into the galaxy and so Kryptman ordered a galactic cordon be established to slow the Tyranids' advance and buy time for Battlefleet Solar and Battlefleet Tempestus to muster. The cordon forced a band of worlds in Leviathan's path to be evacuated and many of them to be razed to the ground to deny Leviathan any bio-resources. Worlds already under invasion were destroyed with Exterminatus just as the Tyranids descended to feed upon the doomed planet, making them expend bio-resources to claim a world only to lose everything upon it. Kryptman's cordon condemned billions of Imperial citizens to death - the single largest act of genocide upon the Imperium since the Horus Heresy. His actions were met with outrage by many influential Inquisitors. He was labeled a radical, a traitor, a fool and the Inquisition ultimately issued a Carta Extremis that stripped him of his title and reduced him to a criminal. But his cordon did succeed in slowing Leviathan's advance to a crawl.
The Octarius War
Kryptman, despite being cast out of the Inquisition, decided to harness the Tyranid swarm and turn it against the galaxy's other enemies. He planned to use a tendril of Leviathan to eliminate the Ork Empire of Octarius, as the Imperium was stuck in an ever-escalating war against Octarius that the Orks were dominating. Kryptman led several specially equipped Deathwatch teams into the caverns of Carpathia to attempt to capture a brood of live Genestealers in a stasis field. The mission was successful, although Kryptman's force suffered heavy casualties. He then planted the brood on board the Perdition's Flame, a Space Hulk that emerged from the warp in the path of Leviathan. As the Genestealers awoke, he destroyed the moon of Gheist in order to divert the hulk's trajectory directly towards the Octarius Empire. Despite the Ork's efforts to quash the Genestealer infestation, a tendril of Leviathan changed course towards the psychic signal of the Genestealers and invaded the Empire. Within weeks, a dozen worlds in the Octarius Empire became infected. Depending on the source, the date of Leviathan's invasion is either 989.M41  or 999.M41 .
Leviathan was nearly stopped by the Orks at Ghorala. When Leviathan's fleet arrived they were countered by every Ork vessel within a dozen light years, as well as extensive mine fields hidden amongst Ghorala's asteroid belt which tore Bio-ships apart in cataclysmic explosions. Ork gunships assaulted the surviving vessels before they had time to react and the Tyranid fleet was all but destroyed. However a single bio-ship managed to break through the Ork blockade and hurl itself at Ghorala, heedless of pain or injury. It launched thousands of Mycetic Spores onto the planet's equatorial planes. On the planet's surface, the small Tyranid swarm cunningly adapted its tactics in order to slowly but surely gather more bio-resources and increase their numbers. When Skarfang, the Ork's leader, was killed by Lictors, the Orks on Ghorala collapsed into infighting and became easy prey for the Tyranids. Each tribe was isolated and destroyed in quick succession until, within weeks, all the Orks on Ghorala had been killed. The bio-resources of the planet were turned into new bio-ships and the Tyranid infestation quickly grew again and spread to nearby Ork worlds. The war eventually spread to Octarius, the central world of the Empire. Never before had Octarius seen such bloodshed, and the war soon spread from the planet out to the entirety of the Octarius Sector. Other Ork worlds lost to the Tyranids include Orrok, Derragon and Keltor.
The Octarius war contines to escalate. Recent reports indicate that the Tyranid Swarmlord has joined the assault on the Orks of Octarius. Kryptman's plan to have both xenos species destroy each other at first appeared to be successful. However, his plan backfired. Orks from lightyears around are flocking to Octarius, and are growing bigger and stronger on their diet of conflict. And every Ork devoured gives more biomass to the growing Tyranid swarm, which is constantly learning and adapting new ways to defeat their foes. Kryptman started a series of events that is making both sides stronger. Whichever race wins the war will emerge stronger than before.
- Tesla Prime - A major Adeptus Mechanicus weapons testing facility. It was in Kryptman's cordon and was evacuated. While the Mechanicum attempted to remove as much material as possible some equipment proved to large, or to dangerous to move and so was abandoned. Shortly after the last ship departed Tesla Prime Ork looters proceeded to raid whatever had been left. The explosions from their 'experiments' could easily be seen from space. When Leviathan arrived the Orks had managed to master many of their new toys resulting in massive casualties on both sides and causing the conflict to spread to many nearby worlds. The resulting war proved to be extremely helpful to the Imperium allowing them to fortify or reclaim many smaller strongholds against further incursions. Kryptman, at this point no longer part of the Inquisition, monitored the events on the planet through the troubled dreams of Astropaths still loyal to him. The discovery that both xenos species were exacting huge losses upon each other led to his discussion to start the Octarius War.
- Baal - Leviathan threatens the homeworld of the Blood Angels, resulting in much of the chapter and its Successors mobilizing for war.
- The Battle for Kalevala - The homeworld of the Steel Confessors comes under assault.
- Rigant - A once-preaceful planet of golden plains and little else. The war from Tesla Prime spread to the planet.
- Gryphonne IV - A Forge World located within Kryptman's cordon but the Adeptus Mechanicus refused to abandon their planet, one of the principle forge worlds in the galaxy, to Exterminatus. Alone in an abandoned system, they prepared for war. The planet was the home of the Legio Gryphonicus, fearsomely disciplined Skitarii legions, and could boast to have the finest defences of any world in the galactic south of the galaxy. However, within days, the planet was overrun by Leviathan; its Skitarii overwhelmed by hordes of Gaunts and the Legio Gryphonicus torn asunder by Tyrannofexes and Hierophants.
- Gorgo - In 996.M41 a tendril of Leviathan, designated 'Gorgo's Talon', invaded the planet of Gorgo. Despite the Cadian 23rd Lucky Dogs garrisoned on the planet, it fell to the Tyranids within a single week. Captain Sicarius arrived at Gorgo just as every living thing was being rendered down to bio-resources. He launched a devastating counter-invasion using the Spear of Sicarius formation to strike hard at the swarm's Synapse creatures.
- Sondheim V - Leviathan descended upon Sondheim V just after M'kar the Reborn had transformed it into his private pandemonium. Carnifexes battled Bloodthirsters in the streets whilst Zoanthropes conducted psychic duels with Lords of Change. The Sky Sentinels Chapter were dispatched to reclaim Sondheim V for the Imperium. But upon seeing the situation they opted to call in Exterminatus instead.
- Forgefane- Leviathan assaults this supposedly impregnable stronghold of the Iron Warriors and consumes the planet in less then a week.[1a]
- St Caspalen - A Missionary World that could put up little defence against Leviathan as its Planetary Defence Force was rendered ineffective by the insidious actions of Deathleaper, a unique Lictor created by Leviathan as the ultimate weapon of terror.[1b]
- Stormvald - A Tyranid swarm was singlehandily defeated by the Eldar Phoenix Lord Maugan-Ra.
- Forgefane - A supposedly impenetrable Iron Warriors Fortress World which fell to Leviathan after Trygons tunneled under the fortresses.
- Ulik Sector - Space Marines from the Death Strike and Genesis Chapters undertook a series of Exterminatus missions on worlds in the Ulik Sector before Leviathan had a chance to consume them.
- Battle of Bloodstar - Admiral Trankar of the Ultima Fleet ends his disastrous campaign against Leviathan by allowing his fleet to be ambushed by two seperate tendrils.
- Valedor - A world populated largely by pilgrims and monks. It was overrun in a matter of hours.
- St Capilene - An Ecclesiarchy controlled world that held out for weeks under the Sisters of Battle. The world fell a handful of days before the Tyranids were driven from Tarsis Ultra.
Leviathan continues its assault the galaxy. Two dozen Space Marine Chapters are known to be involved in containment operations, with Captain Aajz Solari of the Raven Guard in overall command.[6a] As of 997.M41 Imperial Guard Army Group Twelve, comprised mainly of Catachan and Jornian forces, continues to hold out against a tendril as reinforcements are being raised. In 999.M41 a tendril of Leviathan has been identified to be on a direct course to the Blood Angels' Homeworld of Baal.
The creatures of Leviathan display a recognisable colouring of white skin with purple carapaces, and usually red weapon symbiotes.[1c]
- 1: Codex: Tyranids (5th Edition), p.28-31
- 2: Codex: Tyranids (4th Edition)
- 3: White Dwarf 362 (UK) - Liber Apocalyptica: Trygons article, p.46
- 4: Planetstrike, p.30, 57-59, 74-77
- 5: Codex: Blood Angels (5th Edition), p.17
- 6: Codex: Space Marines (5th Edition)
- 7: Codex: Imperial Guard (5th Edition), p.27
- 8: Codex: Orks (4th Edition), p.29
- 9: Warriors of Ultramar (Novel) by Graham McNeill
- 10: Planetkill (Anthology)
- 11: Codex: Blood Angels (5th Edition) pg.16-19