Mortarion
Also known as "Death Lord", Mortarion, Primarch of the Death Guard (XIV legion), now Chaos Space Marines, was left on the plague planet Barbarus by the Chaos gods. He was found by the Overlord of Barbarus, who may or may not have been a Daemon. He may have been a sorcerer of Nurgle, but whatever he was, he was a something that was less human than Mortarion. The Overlord, naming him Mortarion (to mean "Child of Death") took the infant Primarch and raised him, teaching him the arts of combat and warfare. Mortarion's curiosity led him to leave home and set up shop in one of the human villages in a less plague-filled valley, where he simply helped with the harvest. When the village was attacked by a marauding warlord, Mortarion charged into the fray, defending his home with his scythe. After the battle was won, Mortarion taught the villagers, and other humans on Barbarus, the same lessons of warfare he had learned from his adoptive father. Soon, they were strong enough to take the fight to the warlords of Barbarus until only the Overlord remained. The Emperor showed up in a modest robe, as he deigns to do, and challenged Mortarion to slay the Overlord alone or swear loyalty to him.
Mortarion wasted no more time and went to kill his adoptive father, but he was too weak and succumbed to the poisonous upper atmosphere. Cue Emperor time and one hit kill. The Emperor stole Mortarion's kill and evil adoptive father. 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-batman. He wasn't that bad of a guy.
When the Horus Heresy began, Mortarion was like "Yeah I won't worship Chaos, but I'll follow you anyway!" with Horus; however, that wasn't as easy as he thought, for when his big ol' fleet was pimping their way to Terra, they were caught in a Warp Storm, where they had to suffer some diseases and mutations, suffering so hard that it makes a Dark Eldar torture feel like massage (no, really). Well, what would be the best way to stop the disease? Start the worship of the God of the Diseases, of course! To be fair, Typhon was already doing that- in fact, he was the one who arranged them to be caught in the warp storm in the first place. Nurgle was pleased as the Death Guard swore loyalty to him, and ended their suffering (but made them walking sacks of meat that simply won't die, our beloved Plague Marines).
After Horus got his ass kicked, Mortarion made epic formations, and marched his ships (yes, marched them!) to the Eye of Terror, where they found a nice new home, known as Plague Planet, which they started decorating and uhh... "cleaning".
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 Daemon 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.
To his credit, he did make himself useful in the Fall of Sanctia in 437.M36 by sending an army of diseased Orks to soften the planet up, then made their corpses explode into a massive horde of Nurglings when he and the Death Guard landed, wiping out all life in less than a day. (Thanks, Phil!) Now if only some other assholes would do some work every now and then...
The Draigo Incident
Tl;dr: he got his ass handed to him by Kaldor Draigo who carved the name of the previous SUPREME GRAND MASTER, Geronitan, into Mortarion's heart.
The Longer story as told by the Audio Drama Mortarion's Heart, Geronitan more or less baited Mortarion and allowed whole sectors to die under the Primarch's massive force (every Nurgle cult and warband within 100 sectors) until he showed up at Kornovin, which is one of the few places the Grey Knights can preform the ritual to bind his soul and kill him for good. It failed due to the face Geronitan forget to Wear a helmet so Mortarion just Plunge wind him to death and laughed as the entire Grey Knights Chapter cried (yes this happened.)
After Kaldor got elected, mostly because he's the most expendable Grand Master who's also skilled enough to fight the Death Lord. Grand Master Crom gave Kaldor the true weapon, the name The Emperor gave to Mortarion.
After a laughably short and one sided fight, which was really just Mortarion beating Kaldor into the ground, Kaldor was able to light Mortarion's cape on fire with his pysker powers. Mortarion still being a big baby about magic (despite being a psyker himself now) has a freak out allowed Kaldor to slip Mortarion's true name into the Death Lord's mind, resulting in him EXPLODING!! Kaldor then writer's Geronitan name on Mortarion's Heart since its would weaken the Primach due to it being a Killing Word design to hurt Mortarion.
On the Tabletop
And then Forge World gave him rules!
Mortarion is a tough SOB with the Primarch rule, T/W 7, flat dice wounds like poisoned and fleshbane wound on 6's, FNP, rerollable IWND and Toughness tests, 2+ armor save and a 4+ invulnerable save, he's basically a Gargantuan Creature for all intents and purposes. Death Guard gain Stubborn and Poisoned (4+) on all frag grenades and missiles which is pretty sickening. For weapons he has the Lantern pistol which is basically a pistol meltagun, phosfex bombs, frag and krak grenades and Silencer, a S8 power scythe with sweep attacks that causes Instant Death.
Not enough for you? Good, because they also gave him the Shadow Of The Reaper rule. It makes enemy units take a fear check at -1 leadership. Awesome. But much more importantly in the shooting phase if Mortarion, while not locked in combat or embarked, does not shoot or run he can take a leadership test and teleport 10 inches away from his current position. It does not count as moving, and does not prevent him from charging. There are some other caveats, but he effectively has a minimum 18" charge range without factoring in his base size. Because he's fucking terrifying.
| 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 |