Necrosius

Before he was the Undying, Necrosius was an apothecary of the Death Guard, sworn to preserve the lives of his battle-brothers. When the Legion fell to the corruption of the Warmaster during the great betrayal, he turned that learning inside out. Casting aside the healer he had been, he gave his rotting body and blighted soul wholly to the lore of death, and set himself to master the dark sorceries that his gene-father's Legion had come to embody.

He found in Nurgle's gift not merely disease but a grim promise of eternity. Necrosius came to believe that the Grandfather offered a deathless existence to all Mankind, a release from mortality bought with rot and undeath, and he made himself an evangelist of that terrible salvation. Around him gathered a following of like-minded warriors, converts drawn to his vision of a galaxy remade in living decay, and from them he forged the warband known as the Apostles of Contagion, of which he remains the master.

His signature art is a foul necromancy that others name the Zombie Plague, a working of sorcery and disease that raises the freshly dead into shambling, mindless half-life. Loosed upon a battlefield, it turns the slain of both armies into a swelling tide of walking corpses that overwhelms the living by sheer weight of rotting flesh, and Necrosius directs this horde with the cold detachment of a physician observing a spreading infection. The Apostles are held to blame for the outbreaks of the walking dead that scourged the armoured world of Vraks during its long and terrible siege, where the dead rose faster than the defenders could burn them.

It is the manner of his survival, though, that has made his legend. Necrosius has been marked for death many times over, hunted by the assassins of the Imperium and by rival champions of Chaos alike, and among those said to have sought his end is the arch-sorcerer Ahriman of the Thousand Sons. Each time he has been struck down, and each time he has risen again from apparent destruction, reconstituting his blighted form and returning to his work as though death itself could find no purchase upon him. From this the Legion named him the Undying, and few who have faced him doubt that the title is earned. He endures as a testament to his own creed, that in the garden of the Plague God death is never an ending, but only a change of state.