Barbarus

Barbarus is a world drowned in death. Perpetual banks of toxic fog roll across its surface, thickening with altitude until the high peaks are shrouded in lethal, corrosive miasma that no unprotected lung can breathe. In this poisoned dark, humanity clung to survival in the deepest valleys, huddled beneath the killing clouds, while the heights were ruled by monstrous warlord-sorcerers who preyed upon the folk below like gods of cruelty.

Into this misery fell a child from the stars, taken and raised by the greatest of those overlords upon the highest and most poisonous peak of all. That child was Mortarion, future primarch of the Death Guard, and he grew hardened against the toxins that would kill any normal man, climbing ever higher into the deadly fog until he turned at last upon his cruel foster-father.

Mortarion's war to free the human tribes from their overlords shaped him utterly, teaching him a grim resilience and a bitter distrust of sorcery that would define his Legion for millennia to come. The people of Barbarus, who endured a world that sought constantly to kill them, foreshadowed the terrible endurance of the warriors who would one day bear their primarch's name.

It was upon this world that the Emperor found His lost son during the Great Crusade, reclaiming him for the Imperium and the stars. Yet the lessons of Barbarus never left Mortarion, and when he fell to Chaos the daemon primarch would trade one form of poison for another, embracing the plagues of Nurgle as he had once endured the fog of his homeworld. Barbarus taught its greatest son that survival is the only virtue, a lesson he never unlearned.