Demon lords in Dungeons & Dragons are the ultimate embodiments of evil in the multiverse. They each rule at least one layer of the Abyss, which is where the evilest of evil spirits go after they pass on. These include monsters like Baphomet, the prince of beasts, Dagon, the ruler of monsters of the deep, and Graz’zt, the patron of tyrants and despots. Then…there are these guys.
Like the Roman pantheon, which had dozens of mundane gods, D&D has a demon lord for just about everything. Did you know that mushrooms and mucus could be inherently evil? Well, they can in D&D. Then again, this is a game that literally has monsters that disguise themselves as floors, so you shouldn’t be that surprised.