Mortarion

Raised amid the choking mists and predatory horrors of the death world Barbarus, Mortarion clawed his way to mastery over that hostile environment long before the Emperor's arrival, and the resentment he carried toward his absent father for that abandonment never fully faded even after he was recognized as a primarch and given command of the XIV Legion. His towering scythe and his gaunt, skeletal bearing became fitting symbols for a commander whose entire worldview had been shaped by a childhood spent staring down death and refusing to yield to it.

The virus bombing of Istvaan III, which slaughtered so many of his own loyal sons even as it targeted them as traitors, shattered whatever fragile trust Mortarion still held for the Emperor's Imperium, and the toxic ordeal that stranded his fleet shortly afterward delivered him directly into Nurgle's waiting embrace. What emerged from that transformation was no longer entirely the primarch who had once mastered Barbarus through sheer will alone, but something closer to an extension of the Plague God's own patient, rotting purpose, a being who led his Legion through the remainder of the Heresy consumed by a bitterness that Nurgle's gifts only deepened rather than resolved.