Event · M41–M42
Hive Fleet Leviathan
Hive Fleet Leviathan is a Tyranid hive fleet whose late-41st-Millennium arrival opened the Third Tyrannic War, the largest tyranid incursion then recorded. Detected first by outriders and bio-ship swarms probing the galaxy’s eastern fringe, its main strength divided into two vast tendrils advancing on separate axes. One bent toward the Tau Empire’s borders and the Octarius Sector, drawing Leviathan splinters into a war against Ork Waaaghs that has since become self-sustaining; the other struck deeper into Imperial territory, consuming agri worlds and fortress systems as it converged on the Segmentum Solar. The gravest of Leviathan’s assaults fell on Baal, home world of the Blood Angels, whose Chapter and successors fought a defence of near-annihilation against the fleet’s splinter in 999.M41. Reinforced by allied Chapters and Imperial Navy battlefleets, the Blood Angels held the Baal system at ruinous cost in an action recorded as the Devastation of Baal, even as the opening of the Great Rift severed the sector from wider Imperial support at the height of the siege. Elsewhere, other Leviathan tendrils harried the fringes of Ultramar and the Damocles Gulf, forcing local commanders to meet the hive fleet without the coordinated response a threat of its scale demanded. Leviathan was never destroyed as a single body; it fragmented into splinter fleets that persist into the 42nd Millennium, most enduringly in the Octarius Sector, where Imperial strategists have chosen to contain rather than end its war with the Orks. Its passage is counted among the opening calamities of the Era Indomitus, alongside the Great Rift and the Noctis Aeterna, and its surviving broods are cited whenever Imperial commanders assess the tyranid threat along the galaxy’s eastern approaches. Accounts of its full extent remain incomplete, since many worlds it consumed left no survivors to report contact.
See where Hive Fleet Leviathan occurred on the galactic map in the 41st Millennium.