By the time Mortarion's incubation pod crashed on Barbarus, the Warlord and his ilk of similar creatures had already risen to considerable power and was considered the mightiest of his class on the planet. The Warlord was reveling in a recent victory until the silence was shattered by the scream of a child. It is said this warlord walked the battlefield for a day searching for the child, not stopping once until he found it. For a moment he considered killing the child, but he realised that no human should be able to breath at this height, let alone cry out. He considered what he had found, and then bundled the child up and carried it from the carnage. He now had a son, something he had craved for years despite his dark magical powers. The warlord christened the child Mortarion, child of death.
The warlord tested how high the child could survive in the poisonous atmosphere of Barbarus and then erected a massive wall of black iron. He then moved his mansion past this to keep it from the child. Perhaps he knew the child was better than him and that one day he would come for the warlord, or perhaps he was afraid of the small child able to breath where no other of his kind could. Whatever he felt, he trained the child in his image. He taught everything of warfare to Mortarion. He was constantly at the front fighting against all of the other warlords' armies, sometimes of undead humans, sometimes of more daemonic creatures. Mortarion was still human though, and he sought to know of those who dwelled below the layer of fog. Eventually Mortarion escaped from his holdings and descended the mountain, the warlord bellowing after him of his treachery and that to return would mean death.
Mortarion ventured into the lower depths of Barbarus, where he eventually became the leader of the miserable peasants who dwelt there. Horrified by how they lived, Mortarion led a revolt against his tyrannical father. Mortarion's armies won many victories, and soon only the top of the mountains denied him access. After hundreds of battles and wars, there was just one inaccessible mansion, one which Mortarion knew well, the one where his adoptive father resided. As the final battle approached, Mortarion was approached by a bizarre stranger. The stranger challenged the young primarch to capture the last mansion alone, but if he failed he would join the stranger in total obedience. Mortarion turned away and began the ascent to the final mansion, that of the man he had called father, alone. He marched to the top with the anger given by years of building hatred for the final warlord. He climbed higher than he had ever gone before, ignoring the increasing toxins.
When the confrontation came, it was mercifully short. Even the hoses of his suit began to corrode and rot down and Mortarion was gasping for breath. The last thing he saw was his adoptive father, the Warlord of Barbarus, walking towards him to fulfill the promise he had made years before. Then, the stranger stepped between them and, defying the fog, killed the warlord with one mighty sweep of his sword. The stranger then revealed himself as the Emperor of Mankind, Mortarion's true father.
Sometime after becoming a Daemon Prince of Nurgle, a vengeful Mortarion managed to find his fathers soul in the Warp. Relishing in his chance for retribution, Mortarion had the soul trapped into a gigantic clock-like device that continually tortured the Warlord with poisons and diseases. Mortarion put the device aboard the Endurance and would occasionally talk to it, and promised his father that after he was done destroying the Imperium that he could finally pay the tortured soul proper attention.
- 1: Lantern's Light (Short Story)
- 2: Index Astartes: Death Guard
- 3: The Horus Heresy Book One - Betrayal, pgs. 122–126
- 4: The Lords of Silence (Novel) — Unification