Only War: Eventide of Eleusis
Hive Fleet Dagon
The Tyranid attack into the Jericho Reach has left the forces of the Imperium confused and disordered. In part, this is because the Tyranids spread a shadow in front of their advance which disrupts the Astropathic communication on which the Imperium relies. Additionally, the Achilus Crusade has no means of knowing how many Tyranids swarm in the void towards the rim. This confusion has meant that no accurate verified estimation of the strength of the Tyranid forces has been made by the High Command, and both the Space Marines (and more importantly, the Deathwatch) have not shared their findings openly with the Crusade forces.
In an atmosphere of rising panic and little information, some have claimed that the Tyranids must have vast numbers, while others judge that there is no evidence that supports anything other than an attack by a small splinter fleet, numbering perhaps a score of bioships. Most examining the unfolding situation, however, agree that they are facing a splinter of Hive Fleet Behemoth which became detached before or during the breaking of the great hive fleet in 745.M41.
Although some amongst the Magos Biologos, Ordos Xenos and Administratum disputed the correctness
of attaching a new designation to the Tyranid forces facing the Jericho Reach, such a designation was given on 989814.M41 by Lord Militant Tetrarchus. Once, it may have begun as a splinter of a greater fleet, but it was now a threat in and of itself—a threat of the present, not the victorious past. As such, it was given a name from ancient times that spoke of dooms and monsters dreamed of in an archaic age.
That name is Hive Fleet Dagon.