| Category | primordials |
| Type | sentient spores |
| Rank | Primordial Lord |
| Location | unknown |
Mephirouth was a primordial lord of the Creation War. Though he held that title, he was more interested in perfecting his study of medical science than aiding the primordials in defeating the Nawirrûs Covenant. During the fighting in sector Narbuzad, Mephirouth and his retinue of plague 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 enhancing medical advancement by studying the effects of diseases on the sick, wounded, and healthy - all engaged in fierce combat on the surface above. His minions often served as hosts for these diseases, tasked with closely interacting with subjects to infect them. Mephirouth cared not whether the subjects were supposed primordial allies or Covenant enemies, as it was all in the name of medical progress.
Crulock was established in 8702 DE as a field laboratory. It expanded in scale as additional subjects were transported there, and defensive measures were subsequently implemented to address the annoyances emanating from the surface, where efforts were made to eradicate the "disease factory" that frequently resulted in more casualties than those sustained in combat.
Mephirouth was highly unconventional. He constructed Crulock Globes, gelatinous spheres containing nature energy, solely to expedite the growth of the fungi gardens surrounding Crulock.
Mephirouth remains alive despite numerous previous demises, possessing the remarkable ability to regenerate from any host afflicted by disease. Regrettably, this process claims the life of the host, their body becoming the new vessel for Mephirouth's consciousness. He has never aspired to attain the status of a Higher Power, considering such matters insignificant in comparison to his scientific advancements. Mephirouth was presumed deceased until it was discovered that he had instructed Rioch Tetrax on disease-related matters. When Rioch Tetrax ascended to Cult Power with Mephirouth's aid, he adopted Pestilence as his Divine Domain.
The master of disease, Mephirouth, has engineered a number of vessels for pestilence. He endowed groups of rats with cunning, giving rise to the initial rat swarms. His most formidable creations are the plague spewers, fashioned by infecting giants with a mutating affliction. These giants were warriors that he commanded under Piranoth. When his focus intensified on the study of disease, he transformed these soldiers into plague spewers.