Barbarus

With an ugly-ass planet like that. No wonder Mortarion grew up so damned fatalistic.

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. It's constantly dark as the toxic fog blocks out the light of the dim sun that the planet orbits. Humans can only live in the low altitude plains as everything higher than that is a deadly fog where the planet's overlords live, enslaving and treating the peasants in the plains like shit. The fog was so bad even Mortarion couldn't live high up on the mountains! There's not much on Barbarus to see, really, aside from the Wall of Memory that 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 planet's 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 planet's Overlord spared and adopted the crying baby Primarch. The Overlord then trained the Primarch in the art of war and built a mighty wall at the highest elevation that 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. Mortarion 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 where he couldn't survive. 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. Typical Emperor douchebaggery. He then gave Mortarion the Death Guard (then Dusk Raiders) and told him to fuck off and claim the galaxy in Emp's gold-plated name.

It was the first primarch homeworld to be destroyed by the Dark Angels when they went on their rampage behind the traitor lines during the Siege of Terra, but not the first to be destroyed; that honor goes to Nostramo, which got blown to bits by the Night Lords prior to the Heresy.