Barbarus

Barbarus was a death world where the pod of Mortarion landed and was the homeworld of the Death Guard legion. Living on Barbarus is pretty shitty. Its constantly dark as the toxic fog blocks out the light of the dim sun the planet orbits. Humans can only live in the low altitude plains as everything higher than that is a deadly fog where the planets overlords live, enslaving and treating the peasants in the plains like shit. The fog was so bad even Mortarion couldn't even live high up on the mountains! Though to be fair, I'd rather live on Barbarus than South Sudan. There's not much on Barbarus to really see aside the wall of memory which holds a nail with the name of every fallen Death Guard during the Great Crusade.

History

Before Mortarion arrived on Barbarus, the humans there were enslaved and dicked with by the planets overlords who poked the humans with their pointy sticks then hid in the super toxic fog until they decided to poke them again. After Mortarion crashed on Barbarus, he was found crying up on the mountains where the planets overlord spared and adopted the crying baby primarch. He then trained the primarch in the art of war and built a mighty wall at the highest point Mortarion could withstand without being poisoned to death. Mortarion then buggered off to the plains and decided to live with the most similar things to him. Though it is quite a stretch calling a primarch similar to a human, they were closer to him than he was to his adoptive father - Gathering Storm confirmed that Foster daddy was an alien.. He just showed up and helped with the harvest until the undead armies of the overlords showed up. He killed them all except for the ones on the one peak that he couldn't survive on. Mortarion then led a crusade against his father but had to be saved by the Emprah who stole Mortarion's kill after Emps double-dared Mortarion to kill Morty's foster father. Average Emperor douchebaggery. He then gave Mortarion the Death Guard (then Dusk Raiders) and told him to fuck off and claim the galaxy in Emps gold-plated name.