Rovagug

Imprisoned since the Age of Creation, the god Rovagug (pronounced ROH-vah-gug) seeks only to destroy creation and the other gods. Believed to be imprisoned in a state of torpor somewhere deep within the world, his increasingly restless stirrings are taken by many to be the cause of volcanic activity and earthquakes.
History
In a time so long ago Rovagug gnawed his way out of the deepest Abyss and sought to destroy the world. During the War with the Elemental Titans he saw his change. Even then an ancient being, Rovagug was responsible for the destruction of countless worlds.
When he turned his gaze upon a new world he was opposed by an unlikely collection of gods: Abadar, Asmodeus, Calistria, Desna, Erastil, Gozreh, Pharasma, Sarenrae, and Torag, along with a number of other gods from more remote parts of the world. Many gods died in this battle, but their names have been forgotten, certainly the gods who sided with the Rough Beast will never be remembered.
Unable, or perhaps unwilling, to destroy him, the goddess Sarenrae sliced open a hole in the Material Plane, and Asmodeus bound him with a key only the Prince of Darkness held. This prison, known as the Dead Vault.