The world of the eternal night. The Mordian day and year are the same length, the planets rotation taking the same time as its circumnavigation of its sun. Therefore while half the planet is burnt by the harsh sun the other half enjoys the eternal night. Mordian is an extensively over-populated world, with mile high, spires and multi-level towers housing the populace. A world one tenth the size of earth with ten times the population, the over-crowding and harsh living conditions provide a ripe situation for rebellion.
It takes maximum effort by the planetary government to maintain order, and ensure some discipline allowing the populace to survive although not many prosper. The Iron Guard is the instrument of order in this anarchy. Their iron will and rigid disciple is all that stands between anarchy and collapse of the social structure and the continued survival of the population.
Although minor rebellions were common and were put down rapidly, one hot summer a group of powerful individuals made a pact with chaos, casting a spell to increase their powers. As the spell was completed, chaos marines poured out from the Eye of terror, accompanied by daemons and other vile warp creatures. They rampaged through Mordian while the iron Guard fought back valiantly. Slowly the Guard was pushed back, and made a strong stand in their capital city. Evry building and structure becoming a fortress in their own right. The rigid fire-discipline of the Iron Guard took a heavy toll on their attackers for every metre of ground loss.
But the enemy was too numerous; too strong and the Guard finally stood on the steps of the imperial palace to make their final stand. The tide turned! As the time that the Mordians had bought with their lives and their dogged defences, had allowed imperial psykers beyond the robit of their planet to cast a counter-spell and the forces of chaos dissolved before their eyes, being pulled back in to the warp, as the mystical energies that maintained them in the material world dissipated.
Mordian Regiments are highly valued for their stubborn will and iron discipline.
(Ref: Codex Imperial Guard, 2nd ed; Page 13)