Doctor Festus i presume.
Festus the Leechlord is a villainous and grotesque mad doctor from Warhammer Fantasy Battles and a leader of the Nurgle division of the Hordes of Chaos. Once a kind hearted medic of the Empire, he became one of Nurgle's most loyal and dedicated servants and champions.
Story
Doctor Festus was once a talented healer who founded hospices all across the imperial province of Nordland , and was as compassionate as he was gifted. Specialising in curative unguents, the good doctor cured hundreds of people every year. With Festus’ guidance, Nordland overcame outbreaks of various nasty diseases.
It was the onset of the Gnashing Fever, that marked the beginning of the downfall for Festus. Despite his talents, the doctor could not stop the spread of this new and highly contagious disease. Festus locked himself in his laboratory, working ceaselessly to create a healing elixir. Countless sleepless nights passed, and despite his best efforts, Festus still failed to make a cure.
The victims of the fever he had managed to keep in his laboratory were dying and he was powerless to prevent it. As the last of his test subjects shook themselves to death, Festus dropped to his knees, crying out for help. One by one, the slack-jawed corpses in Festus’ laboratory turned their heads to look at him. With the voice of Nurgle emanating from a score of parched throats, he promised to give Festus the knowledge necessary to cure not only this plague but all the diseases in the world, in exchange for a lifetime of service. In his desperation, Festus agreed.
In the blink of a eye, Festus’ mind was filled with every detail of every sickness, ailment and plague known to the great god Nurgle. This drove him entirely mad, washed away his compassion and left nothing more than an intimate knowledge of disease and a desire to experiment on anyone. Festus became the Leechlord of Nurgle, who goes to war, leading an army of sick and deformed chaos warriors and nurglite daemons in the name of furthering his revolting studies. Though his skills has a healer are greater than ever before, woe betide the fool who crosses the Doctor, for he is always in need of new test subjects, and not above force-feeding his latest concoctions to his victims in his quest to bring ever more repugnant forms of life into the world. It is a better fate by far to die on the field of battle than to be captured alive by Festus and used for his latest dark experiments.