Mordia
Mordian (Used to be called Mordia until GeeDubs changed it into an adjective because of...reasons...but half of /tg/ still calls it Mordia) is a Imperium Hive World that was renowned as the Homeworld of everyone's favourite Commissar army; the Mordian Iron Guard Regiments of the Imperial Guard.
Overview
Mordian is a tidally locked planet; the time of the planet's rotation equal to the time of its revolution around its sun. Consequently, half of the planet is in perpetual darkness while the other is constantly burned by the harsh sun. All life is on its dark side. For good reason, Mordian is also known as the World of Eternal Night.
Mordian's hundreds of millions of inhabitants live on a land surface barely one tenth the size of Earth in an area that real-world astrophysicist call the Twilight Zone, where it is neither too hot nor too cold for life, although the majority of the hives are built nearer to the night side of the planet. This is despite the fact that Hives are entirely enclosed and climate controlled due to the fact most "normal" hive worlds are on a scale from climate destroying pollution to Necromunda. So while Hives may not be possible on the day-side (Mercury at least needed archaeotech to allow it to be settled, though Scintilla has a desert hive so hot anyone on the hive skin is likely to die of a bad tan within minutes unless they have protective gear), logically the night-side of Mordia should be where most of the Hives are. Maybe they were sent a cease and desist by the Valhallans?
Its hive cities are characterized by pyramidal, multi-levelled towers rising like mountains towards space. The government is known as the Tetrarchy (No, not THAT one), and strictly controls and rations the planet's meager resources. Mordian naturally breeds discontent, and the Iron Guard is the only thing that stands between order and total anarchy.
History
Chaos Invasion
The greatest threat faced by Mordian came as a result of a conspiracy among Chaos cultists. Summoned by a spell, ships from the Eye of Terror poured Chaos Marines from the skies and daemons appeared within the streets. In the resulting Battle of Mordian, the Mordian Iron Guard eventually emerged victorious against the invasion.
Fall of Mordian
The planet Mordian along with the entire Stygius Sector was invaded by Chaos Forces led by warbands loyal to Tzeentch. The Iron Guard held their ground as reinforcements arrive, but there were too many heretics and slowly the Imperial defenders are being bled dry. (Didn't help that the dead are being raised as zombies or that the Warp claimed several Imperial ships and an entire planet during the conflict.)
Further reinforcements came in as an Eldar Strikeforce from Craftworld Ulthwé aided the Imperium. Being the cryptic pricks they are, they tell the defenders that they can't win and advised them to leave the sector. (Seeing as the Chaos Invasion is led by the Thousand Sons under Magnus as well as M'kachen, they had a point). Even though the discipline of the Iron Guard kept them from being overrun, they agreed to retreat, and evacuated what they could. It probably stung, but you'll never see a Mordian getting mopey about it.
However, thanks to the perseverance of the Iron Hands and its successor chapters, Chaos forces were finally driven off Mordian and reclamation of surrounding worlds continued in order to secure the adjacent passage through the Great Rift. It's not quite clear if they reinforced the Mordians or joined a Mordian attempt to re-take Mordian or if they did it on their own. Seems likely that the planet was secured due to their arrival instead of evacuating the planet. Speaking of, that the Iron Hands did this proves the Eldar were wrong. Again.
(Though since the Thousand Sons are back to Heresy numbers, 80k Marines, the Iron Hands and Mordians are going to need A LOT of reinforcements otherwise they are going to fail)