Mephirouth was a primordial lord of the Creation War. He held that title loosely though because he was more interested in perfecting his study of medical science than helping the primordials defeat the Nawirrûs Covenant. During the fighting in sector Narbuzad, Mephirouth and his retinue of plaque spewers, rat swarms, and other forces entered sector Khelan from the direction of Andibutt's Plague Gullet. Khelan is the Underdark area beneath Narbuzad, so at a place called Crulock, Mephirouth and his minions set to enriching medical advancement by studying the effects of diseases on the sick, wounded, and healthy - all fighting in great numbers on the surface above. His minions were often hosts for these diseases, and it was their job to get in close with subjects and infect them. Mephirouth did not care who the subjects were, either supposed primordial allies or Covenant enemies, since to him it was all in the name of medical advancement.
Crulock was founded in 8702 DE as a field lab. It grew in size as more subjects were brought in and then defenses added to deal with the nuisances coming down from the surface seeking to eradicate the "disease factory" that often caused more war casualties than those dying in battle.
Mephirouth is still alive. He has been killed many times, yet has the uncanny ability to bud anew from any host that carries a disease. This unfortunately kills the host, with their body becoming the new host for Mephirouth's sentience. He has never sought to become a Higher Power with no interest in matters he deems petty compared with his medical advancement. He was thought dead until it was learned that he instructed Rioch Tetrax on disease matters. When Rioch Tetrax rose to Cult Power, with the help of Mephirouth, he took Pestilence as his Divine Domain.
As a master of disease, Mephirouth has created a number of carriers for pestilence. He gave cunning to groups of rats, resulting in the first rat swarms. His most powerful creations are plague spewers which were made by infecting giants with a mutating disease. These giants were soldiers that he led under Piranoth. When he became driven in study of disease, he turned these soldiers into plague spewers.