The Segmentum Obscurus is the great northern administrative region of the Imperium, a vast swathe of the galaxy whose reaches are shadowed by the presence of the Eye of Terror, the largest warp rift in the galaxy and the mightiest stronghold of Chaos. More than any other segmentum, it lives beneath the constant threat of daemonic incursion and the wrath of the traitor legions.
Because of this ever-present danger, the Segmentum Obscurus is among the most heavily militarized regions in the Imperium. For ten thousand years its defense was anchored by the fortress world of Cadia and the Cadian Gate, the bulwark that contained the horrors of the Eye. Great fleets, fortress worlds, and countless regiments were stationed here to hold the line against the Black Crusades that periodically erupted from the rift.
The catastrophic fall of Cadia struck this segmentum first and hardest, and it was here that the galaxy-spanning warp rift tore open, plunging whole sectors into darkness and cutting off worlds beyond all hope of aid. The northern reaches became a front line as never before, embattled and imperiled along their entire length. The Segmentum Obscurus endures as the Imperium's most haunted frontier, a region where the war against Chaos is not a distant rumor but a daily and existential struggle.