Hive Fleet Leviathan was the third and largest Tyranid incursion into the Milky Way Galaxy in 997.M41 and marked the start of the ongoing Third Tyrannic War for the Imperium of Mankind. Less than five years after the defeat of the Hive Fleet Kraken in the Second Tyrannic War, Lord Inquisitor Kryptman noted the tell-tale signs of yet another Tyranid invasion in 997.M41. He implemented the Kryptman census, burning out dozens of astropaths in the attempt to contact scores of worlds on every fringe of the Imperium. Slowly, the responses - and lack of them - formed a pattern, and the venerable Inquisitor was able to chart the course of the Tyranids' latest Hive Fleet to arrive in the Milky Way Galaxy. Codenamed "Leviathan", this time the Tyranids were approaching from below the galactic plane, attacking from two points spaced wide apart in the shape of a closing pair of jaws, cutting off large portions of the galaxy from Warp travel or astropathic communication. At a great cost in human life, a combined force of Planetary Defence Force troops, two Regiments of the Imperial Guard, the Ultramarines and Mortifactors Chapters of Astartes, and a Deathwatch Kill-team under the command of Inquisitor Kryptman crushed the left "half" of the Leviathan on the world of Tarsis Ultra, destroying the Tyranids nearest to Terra, and reestablishing astropathic contact with the worlds between Leviathan's jaws. The Imperium had won its first victory.
Yet Leviathan continued to carve its bloody path through the Imperium. Perhaps Leviathan's main gain was the vital Forge World of Gryphonne IV, home of the Legio Gryphonicus Titan Legion. Kryptman knew he had to slow down the Hive Fleet's advance to buy time for the Imperial Navy's Battlefleets Solar and Tempestus to muster. With grim finality, he ordered a cordon to be established. Every world within was to be evacuated and undergo immediate Exterminatus wherever possible, thus causing the Tyranids to expend their forces for little gain, the Exterminatus denying them valuable biomass to replenish themselves. With one stark, callous decision, the Inquisitor had inflicted the Imperium's worst act of genocide upon on its own since the Horus Heresy. Kryptman was denounced as a radical and a traitor; when migrating Orks claimed a score of former human worlds, he was stripped of his title and cast from the Inquisition by his brethren.
Yet Kryptman still had loyal allies within the Deathwatch, and incepted a plan to halt Leviathan without the loss of further human life. When a Genestealer infested Space Hulk drifted from the Warp, members of the Deathwatch loyal to Kryptman captured a brood in stasis. Using the brood of captured live genestealers, the Inquisitor engineered a Tyranid invasion within the Ork-held Octavius system, very close to Leviathan's advance. The crowded Ork cities, so teeming with life, were the source of a huge Orkish infestation, and soon drew the entirety of Leviathan into the tight cluster of Ork systems. The plan had worked, and both races are now thoroughly involved with destroying each other. One thing is certain: each race thrives on war, and there remains the possibility that if the Tyranids emerge victorious, the Imperium shall have to face a threat far greater than before, as in a victory, unlike other races, Tyranids effectively lose no military strength or resources, their casualties and still living warrior strains reabsorbed by invading hive ships along with the remains of the conquered worlds, thus every victory has no repercussions, only benefits. This also means that the Tyranids will assimilate vast quantities of strong, survivable Ork DNA that makes Orks such successful survivors and incorporate it into their own bio-constructs. Indeed, worlds on the edge of the conflict have begun to file captures of Tyrands assault organisms larger than ever before. The implications of Kryptman's manipulation of this alien threat are too horrible to contemplate.
A shadowy council within the Imperium called the Strategic Collective has been analyzing all details of the Tyranid invasions, and have drawn a conclusion as stark as it is terrifying: the fleets faced by the Imperium to date are but parts of a far greater whole, and this whole will be arriving at the Imperium's borders within less than a century. They estimate that mobilization levels will need to be increased by a minimum of 500% - effectively including every able-bodied man, woman and child on every world in Segmentum Solar, Obscurus and Tempestus -- to have even a hope of stopping the Tyranids...
- Codex: Tyranids (5th Edition)