Ultima Segmentum

Ultima Segmentum is one of the five Segmentum Majoris of the Imperium of Man, comprising the galaxy’s eastern reaches and the Eastern Fringe.

Adjacent to the Tau Empire across the Damocles Gulf, the segmentum contains Ultramar, the realm founded by the primarch Roboute Guilliman around Macragge, and extends to Baal, homeworld of the Blood Angels. It also encompasses Octarius, where a centuries-long war between Ork Warlord Snagrod’s empire and Hive Fleet Gorgon has consumed entire star systems; Badab, site of the Astral Claws’ rebellion under Lufgt Huron during the Badab War of 901–912.M41; Thramas, contested by the Night Lords and Dark Angels during the Thramas Crusade of the Horus Heresy; and the lesser-documented systems of Vidar and Korianis, also recorded as Aurelia.

The segmentum holds particular significance in the Imperium’s war against the tyranids. Hive Fleet Behemoth was first detected and took its name near the world of Tyran, its advance culminating in the Second Tyrannic War and the Battle of Macragge in 999.M41, where Guilliman’s Ultramarines held the hive fleet’s vanguard. The Tau Empire’s expansion along the segmentum’s eastern border likewise drew Imperial response, most notably the Damocles Gulf Crusade of 742.M41.

As the Imperium’s eastern frontier, Ultima Segmentum faces sustained pressure from both tyranid hive fleets and the Tau Empire, with Ultramar serving as the region’s most stable and heavily defended domain. Since Guilliman’s return and the Indomitus Crusade, Macragge has again functioned as a rallying point for Imperial forces contesting the segmentum’s embattled systems.