Mortarion

He puts the grim in grim reaper

Primarch of the Death Guard (XIV legion), now Chaos Space Marines. Was left on the world Barbarus by the Chaos gods. He cleaved and smote his way to victory and went to kill his adoptive father who may or may not have been a daemon but, was too weak and succumbed to the poisonous upper atmosphere. Cue, Emperor time and one hit kill. Mortarion gets his own legion and swears allegiance to the big E. After a few years of raping and pillaging those filthy Xenos he becomes best buds with Horus and that creepy pseudo-vampire. Long story short, him and his legion now worship Nurgle and owns his personal little planet in the Eye of Terror from which all disease and plagues in the galaxy originate. Of note, he is now a Deamon Prince, so don't piss him off. He should probably work out more so as to get back into shape, but the fact that he just has to be within a few kilometres for you to die from the toxic emanations sort of renders that superfluous.

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Oh and he got his ass handed to him by SUPREME GRAND MASTER DRAIGO who carved the name of the previous SUPREME GRAND MASTER, Janus, into Mortarion's heart.

Ol' Morty was pretty pissed off about that.

The only possible explanation is that Mortarion let Draigo write on his heart. Maybe doing so condemned Janus's soul to Nurgle's possession, or Mortarion is so powerful that he does not care if somebody graffitis on his heart.

And, for the record, Matt Ward's wet dream is Rowboat Girlyman carving his name on Mortarion's heart.

The Primarchs of the Space Marine Legions
Loyalist Corvus Corax - Ferrus Manus - Jaghatai Khan Leman Russ - Lion El'Jonson - Roboute Guilliman Rogal Dorn - Sanguinius - Vulkan
Traitor Alpharius/Omegon - Angron - Fulgrim Horus - Konrad Curze/Night Haunter - Lorgar Magnus the Red - Mortarion - Perturabo