Festus the Leechlord
Festus was once a plague doktor in ye old Empire, helping people survive from plagues and generally being a not too bad 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 a disease that was 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 looking into the plague to the point that he became obssessed with trying to find a cure. He was offered said cure but 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 infathomable knowledge of poxes, plagues and poisons into Festus' mind, driving him mad and sending him spiralling into the chaos wastes.
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. This lasted unto an unspecified point where he found himself in Altdorf's sewer system, setting up home there.
Prior to Archaon selecting the Glottkin to lead his vanguard assault against the Empire, Festus was approached by Ku'gath Plaguefather. The Greater demon emerged from Festus' cauldron with instructions from Nurgle himself to create a spell of such power that it would tear open reality and unleash Nurgle's garden in the mortal plane 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 seiged Middenheim, Carroburg, Talabheim and the Drakwald respectivley. 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 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 severley weaken the defences of Altdorf even as its defenders tried to rebuild and fortify. His efforts also caused plague to hasten 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 Nurgleesque 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 demonic growths to grow rampant and demons 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. 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 distract with an army of plague-riddled demons at their back and began an assault on the Temple of Shallya, slaughtering the meagre forces of the Empire stationed there. Festus himself directed the Greater demon 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) Leon Louceur descended and began a brutal if short lived assualt that seen 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 devasted, Festus lunged at the broken king only to meet Vlad Von Carstein who had come to aid the human defenders, met Festus in single combat. It was a brief assault which seen Vlad 'killed' as Festus spewed a corrosive vomit all over him, melting him away. Mere moments later, Vlad, ressurected by his ring, dived on to the bloated and empowered Festus, impaling him in the heart with a wooden stake. Ironically, Festus at first began to laugh away the wound, but the regenrative abilities locked in his fat flesh spilled forth into the wood of the steak. 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 demons in that quarter began to give ground rapidly to the massive undead host.