Stygius Sector

The Stygius Sector is a division of the Segmentum Obscurus centred on the Mordian system, and gives its name to the Stygius War. Its chart includes the Mordian system itself alongside the neighbouring systems of Dhobash and Prismata, worlds drawn into the same conflict as it spread outward from Mordian.

The war began after the opening of the Great Rift, when forces of Chaos acting in the name of Tzeentch moved to drown the Mordian region in warp corruption. The Iron Hands Chapter, long associated with the region’s defence, refused to abandon it, and the resulting campaign broke out as the Stygius Crusade, one of many theatres opened across the Imperium Nihilus following the fall of Cadia.

Within this crusade, the Iron Crusade emerged as the defining Imperial effort, holding Mordian itself, the night-world of the Iron Guard regiments, against sustained pressure from the Night Lords and the Thousand Sons. The convergence of these two Traitor Legions, one descended from Konrad Curze’s original Legion and the other devoted to Tzeentch, marks Stygius as a sector where older Legion enmities and newer Rift-born incursions meet around a single contested system.

The Stygius Sector remains significant chiefly as the surviving lock on Mordian rather than as a mere administrative designation within the Segmentum Obscurus. Unofficial charts continue to record the Segmentum Obscurus as its parent region, and treat Mordian itself under its own separate entry as the system whose fate the sector’s ongoing war was fought to decide.