Champions of the Titan's Eye
Your angelic memories recall the Demon Plague, a disease (homebrewed, akin to Ghoul Fever but more terrifying in every way imaginable) that almost drove the sentient races to extinction before outsiders intervened and purged it from the material plane. Your group thought to have removed it, and its knowledge, from the Abyss as well, but obviously this is not the case.
Without magic, the disease never goes away. It quickly sucks the life force out of the weak, turning them into plague-carrying zombies. Those hardy enough to survive are slowly driven insane and become twisted, demonic versions of their former selves. In either case, the disease spreads by attack (like Ghoul Fever above).
Even with magic to remove the disease, Harrod noticed (and pointed out) that there was a growing emptiness… a longing for the demonic presence once it was removed.