Mordian

Mordian is a tidally locked hive world: one face baked, the other in permanent night, and all the people crammed into the dark. The Tetrarchy rations everything; the Iron Guard is what stops the hives tearing themselves apart. Chaos has tried the night side more than once, from a summoned invasion to the Stygius war and a later Webway rupture. Discipline is the local religion.