Nurgle, the Plaguefather, also known as the Intergalactic Garbage Bag, is a Chaos God.
Official Canon Edit
As the god of decay and disease, Nurgle has a hand in virtually all lifeforms in the galaxy. The worst plagues are of his direct making, allowing him to spread his influence easily.
During the Fall of the Eldar, Nurgle intervened when Slaanesh attempted to consume Isha, claiming the Eldar goddess for himself. He has kept her locked away in his plague garden where he uses her to test his various dieases. Being the goddess of healing, she always recovers from his infections; if Nurgle is pleased with the results, he unleashes them onto the galaxy. If not, he starts the process all over again.