These humanoid fungal monstrosities result from an unholy pact, made long ago between sorcerers and demons. It is written, in the Book of Caves, that the demons allowed these sorcerers to transcend their mortal forms, but at the cost of their souls. What was left of them melded into the fungus in their secret underground temple, and created the hybrid mockeries of mankind that are the Myconians.
As predators of the humans, the Myconians are feared and hated, condemned to live deep underground, lest they arouse suspicion and reprisals. They make deals with dark necromancers and the spawn of evil gods, always furthering their desire to enslave humanity in its entirety.