Festus the Leechlord
Festus the Leechlord was once a good plague doktor in ye old Empire, helping people survive from plagues and generally being not too bad a guy (at least in that sense, seeing as GW hasn't done shit for his backstory till The End Times for some reason). He eventually came into contact with the Gnashing Fever, a disease so virulent that he had no idea how to stop the contagion from spreading and killing more and more of the patients and people he had spent his life dedicated to helping. Being a highly decorated doctor from Altdorf, Festus took it upon himself to begin researching the plague to the point that he became obsessed with trying to find a cure. As the last of his patients died, he cried out for help. Then things got creepy; one by one, the slack-jawed corpses in Festus’ laboratory turned their heads to look at him. With one voice emanating from a score of parched throats they made him an offer, except it wasn't them, as the speaker was none other than Nurgle himself.
Nurgle's offer was to have all the knowledge necessary to stop the plague as well as ALL diseases. So Festus, being as determined as he was, agreed. It was then that Nurgle thrust all of his unfathomable knowledge of poxes, plagues and poisons into Festus' mind, driving him mad, washing away his compassion and sending him spiraling into the Chaos Wastes. The irony is that Nurgle might not have actually meant to drive him crazy, because Nurgle really is compassionate and loves everyone.
From this point onward, Festus was obsessed with experimenting and creating both new diseases as well as testing those that Nurgle had given him knowledge of. He spent his life seeking new 'patients' to do so with. At one point he returned to the Empire in disguise to cure an Imperial noble of a disease. Though he cured the disease as promised, Festus left the man with ten more, and several epidemics spread through the town. This lasted unto an unspecified point where he found himself in Altdorf's sewer system, setting up home in his old, now abandoned, labratory.
Prior to Archaon selecting the Glottkin to lead his vanguard assault against the Empire, Festus was approached by none other than Ku'Gath himself. The Greater Daemon emerged from Festus' cauldron (who knows how he fit in there though) with instructions from Nurgle himself to create a spell of such power that it would tear open a rift between reality and Nurgle's Garden and let it spill into Altdorf. Festus made about preparing the great spell, sacrificing many souls to his pot as well as working tirelessly while the Glottkin, Gutrot Spume and the Maggoth Riders of Icehorn Peak beseiged Middenheim, Carroburg, Talabheim, and the Drakwald respectively. Festus' machinations beneath the poor slums of Altdorf caused widespread disease as the plague fumes from his spell wafted through the city, striking down the poor quarter and costing the city valuable resources it would need in the fight to come.
However, this did not go undetected. Newly appointed Supreme Patriarch Gregor Martak, at the behest of one of the sisters of Shallya, urged Reiksmarshal Kurt Hellborg to spare some troops to investigate. However, his pleas went on deaf ears and as a result, Martak left the city to find the then-missing Karl Franz. All the while, Festus helped severely weaken the defenses of Altdorf even as its defenders tried to rebuild and fortify. His efforts further hastened the plague which caused Hellborg to look like a tyrant guarding and controlling the food supplies and to quell several minor insurrections from the inhabitants. Festus was a shit-stirrer, indeed (excuse the Nurglesque pun).
By the time the three armies converged upon a crippled and plague-riddled Altdorf, Festus set the spell in motion. A massive vortex ripped up through Altdorf, allowing daemonic growths to grow rampant and daemons to rain from the sky. At this point, the assault began from all three armies and Festus made his move. Nurgle was pleased as fuck with his mortal fat friend so he took away all his tiredness from having worked without pause for months and gave him new strength and powers, and even planned to offer Festus daemonhood, the ultimate blessing to a champion of Chaos. At this point Ku'Gath came into the mortal realm and burst up out of the cauldron, with growths spreading at a disgustingly rapid pace through the slums. Ku'gath and Festus emerged from the sewers in the poor district with an army of plague-riddled daemons at their back and began an assault on the Temple of Shallya, slaughtering the meager forces of the Empire stationed there. Festus himself directed the Greater Daemon to break into the temple, pointing and goading it and the infernal army to despoil the sacred place.
Almost out of convenience (as if GW would ever do such a thing) Louen Leonceur of Bretonnia descended and began a brutal if short-lived assault that saw both the former king's hippogryph a bloody mess, as well as the corporeal form of the Plaguefather sent back to the realm of Chaos. Outraged and devastated, Festus lunged at the broken king only to meet Vlad von Carstein - who had come to aid the human defenders in his bid for Elector Counthood - in single combat. It was a brief assault which seen Vlad 'killed' as Festus spewed corrosive vomit all over him, melting him away. Mere moments later, Vlad, resurrected by his ring, dove onto the bloated and empowered Leechlord, impaling him in the heart with a wooden stake. Festus at first began to laugh away the wound, but ironically the regenerative abilities locked in his fat flesh spilled forth into the wood of the stake. The life-giving energy spread rapidly through him, twisting his form into that of a giant, twisted tree, tearing him apart. With Festus and the Plaguefather now dead, the daemons in that quarter began to give ground rapidly to the massive undead host.
He has since come back in that newfangled setting, and is basically just doing the exact same shit he was doing before and during the End Times.