Demogorgon is a major figure in the Dungeons and Dragons universe and is perhaps the most powerful entity of the Abyss, the Chaotic-Evil aligned realm. He is depicted as a creature with two mandrill heads, reptilian legs and tail and two tentacle-like appendages for arms. His very gaze can cause insanity or a charmed state and his tentacles will rapidly decay organic tissue after touching it. Like all high-powered demons, he can also cast a vast array of powerful magic spells.
In video games, Demogorgon will occasionally appear in NetHack after being summoned by other high level demons. He is one of the most deadly enemies in that game, as he can cause many different status effects simultaneously. He also famously appears in the optional dungeon of Watcher's Keep, in the Baldur's Gate II add-on Throne of Bhaal. Watcher's Keep is meant to seal the Prince of Demons in an eternal prison; the party can choose to ensure the seal continues to operate as normal or they can attempt to defeat Demogorgon's avatar and banish it to the Abyss for a hundred years. The former is the advised course of action, for many reasons.