The One Chained in the Dark

The One Chained in The Dark is the arch-villian of the pantheon of the Five. The Great Calamity led to the creation of the Five, the rise of the One Chained in the Dark, and the eventual resetting of the world. It's said that the Five were part of an Ancient race that was working on great planar technology. They discovered the secret to immortality on an unknown plane. While moving their people to the plane, the Five were involved in accidentally freeing a creature from the places between the planes.
The Five sacrificed themselves, becoming intertwined with the Ayae and pure planar energy and fought the dark creature, destroying much of the Caer in the process. They eventually sealed the Chained One in a dark demiplane, building a great dungeon and shackling the entity the floor.
The Five worked to reset life in Caer, setting the Ayae to work creating a new world. The first creatures were the dragons and krakens, who created the new land. Some of the dragons were corrupted by the One Chained in the Dark and turned on their brethern.
The Chained One continued to machinate in the dark, eventually corrupting the race of Goblins and claiming dominion over the Underdark, creating dark monstrosities that dwell there.