Otherworldly Evil in the body of a imbecile warlock.
Faustus was a warlock of a Great Old One. However, he was more ambitious and evil than his patron had expected. This wouldn’t have been an issue had he not attempted to overpower the GOO, and make him a slave. Of course, this was a dumb idea.
However, the GOO held back from killing him, and instead made him an imbecile, and let him loose. However, this was discovered by an even more alien and greater evil, who took advantage of the old and impoverished Faustus and took hold of his body. He was transformed, and thus Cenodoxus, Fang of the Snake, came into this world.
Using the enhanced powers of Faustus, Cenodoxus has taken over the small kingdom of Luril, and set it to the production of willing slaves. This consists of the pollution of the water supplies with mutagens and magical toxins. The wildlife becomes Awakened and half-undead, the people grow pale and apathetic and rise after their death, and the trees themselves grow tentacles.
For now, few care – few even know of the situation. But, danger grows and evil threatens to spread out of the valley of Luril.