The Mordian Iron Guard is the superbly drilled and accoutred Militarum Regimentum of the Astra Militarum whose constituent regiments hail from the Hive World of Mordian. This bleak world is tidally-locked with its sun, which causes one side of it to always be bathed in perpetual darkness while the other is continuously bathed in its star's radiation, leaving it a scorched and lifeless wasteland. As such, all human settlements on Mordian are located on the world's dark side, and the planet has well-earned its moniker as the "World of Eternal Night."

The people of Mordian are grim and dour by nature, respecting only discipline and duty. Their regiments are fiercely loyal to their cause: the prosecution of the Emperor's enemies. In battle, these highly-disciplined soldiers present perfectly formed ranks of troops to the enemy, unleashing precisely timed volleys of las-fire from behind a hedge of bayonet points. Some enemies of the Emperor have been misled by the Mordians' elaborate and ornate uniforms to believe they were facing amateurs, only to find the bright uniforms contain tough, steely-eyed and implacable warriors of the Imperium.

(40k Wiki stuff above^ A list of noteworthy Mordian Regiments to be added below at a later date)