The Imperium classifies Tyranids by the Hive Fleet from which they spawn. If historians of the Adrilles War follow that same logic then it would be correct to say the Tyranids involved in the war are Hive Fleet Leviathan Tyranids. In 997.M41, Hive Fleet Leviathan threatened the Imperium and laid waste to hundreds of worlds. Adrilles was one such world.
Though they would perish in the effort, Imperium troops on the moon Brache managed to sever the psychic link between the Tyranid ground troops and the Hive Fleet. How this was accomplished has been lost to history because no Imperium troops survived the Tyranids razing of Adrilles and its moons. Reports taken from support personnel involved in the civilian evacuation of Adrilles suggest that the Tyranids responsible for Adrilles’ downfall were spawned by a single Hive Ship and not the Norn-Queen of Hive Fleet Leviathan. If this was indeed the case then the Hive Ship would have to have been destroyed.
Separated from its Hive Fleet, the Tyranids on Adrilles and its moons would have eventually died out. When native bio-mass was exhausted, the Tyranids’ monstrous creatures would have resorted to consuming its own lesser lifeforms for sustenance. Reclamation Pools and Capillary Towers would continue to operate but unable to feed bio-mass back to the Hive Ship these, too, would eventually fall quiet.
The discarded bio-mass and memories of the Tyranids who fought on Adrilles and its moons lay dormant until stirred to life by the machinations of Magister Militum Frans Ruldolf and his Jager Corps when they settled on Adrilles many years later.
The Jager Corps experimentation was not immediately successful. Some Psykers died from psychic backlash; early Tyranids were not viable; others could not be controlled. It became apparent to Grand Librarian Hoeth that his psykers needed something through which psychic abilities could be channeled and focused. Gates to the ethereal were opened and kept open by select psykers and it was through these gates that other psykers were able to direct, amplify, and focus the psychic energy required to successfully give rise to Tyranid life from the dormant Capillary Towers and Reclamation Pools.
The exact numbers raised and, more importantly, the detailed process of how the raising was achieved is lost to history. Many historians believe the Jagers purposely kept few records or that their processes were recorded in a kind of code, the key two which was lost during the war. Other historians believe the records were purposely targeted by those who sought to stop the Jagers– not just the Vlka Fenryka but also the forces of Khorne. There is, however, a small, fringe group of academic thinkers who believe the Jagers’ records were saved and are now in the hands of the Cult Mechanicus.
What is known for certain is the Sautekh Necrons obtained an artifact– Tyranid biomass fused to Sautekh Necron technology by the corrupting forces of Warp energy– during the Battle of First Contact. This artifact was taken to the moon Grimmig where the Sautekh Necrons were able to use it to exert a tentative control over the Tyranids that still lived in the sunken, damaged tombs below the surface of the moon.
As one historian noted, “The awoken xenos were not willing participants in the Adrilles War in the same way that its other combatants were. Even the Orks with their limited participation had more cause to fight then these descendants of Hive Fleet Leviathan. No, these xenos were nothing but weapons to be wielded to push the agenda of others.”
It is perhaps needless to record that the historian who published such thoughts was ostracised by his peers for doing so.
Notable battles of the Adrilles War in which Tyranids were participants: