Event · M41–M42
Fourth Tyrannic War
The Fourth Tyrannic War is the Imperium’s name for the invasion by Hive Fleet Leviathan, which struck its eastern territories from late M41. Imperial Navy and Adeptus Astronomica observers recorded the fleet’s approach from beneath the galactic plane, and the Imperium came to treat it as the fourth major Tyranid incursion after those associated with Hive Fleets Behemoth, Kraken and Gorgon. Orrery anchors the conflict to Segmentum Pacificus, though accounts differ over how the wider theatre is divided between the Imperium’s segmentae, and Leviathan is recorded as the largest Tyranid force yet catalogued. The cause is the hive mind’s continuing search for biomass, which Leviathan drew from worlds across its advance. The course of the war is recorded as a chain of world-by-world invasions rather than a single campaign. Leviathan’s vanguard organisms, followed by its main swarms, stripped populations and planetary resources as they moved inward, and the Imperial Navy and Planetary Defence Forces were repeatedly forced into delaying actions. Chapters of the Adeptus Astartes, most prominently the Ultramarines under Marneus Calgar, took part in the defence of Ultramar, the Ultramarines’ realm and the setting of the earlier Battle of Macragge in the First Tyrannic War. Tyranid commanders such as the Swarmlord are associated with the fleet’s later assaults, and the Imperium’s Inquisition and Ordo Xenos advised on strategy against a foe whose capabilities were poorly understood. The war’s aftermath is inseparable from the changes at the turn of the forty-second millennium. The opening of the Great Rift and the launch of the Indomitus Crusade followed closely on Leviathan’s advance, and Imperial records describe the fleet as fragmented into multiple splinter fleets rather than destroyed. Later tradition holds that the war continued into M42 as a running conflict, with Leviathan’s remnants still contesting Imperial holdings. Its place in the timeline links it to the earlier Tyrannic Wars, to the Ultramarines and Ultramar in the map graph, and to the wider crises of the Time of Ending.
See where Fourth Tyrannic War occurred on the galactic map in the 41st Millennium.