Tiamat

In the Dungeons & Dragons role-playing game, Tiamat is a powerful draconic goddess, usually depicted as a European dragon with multiple heads. The name is taken from a goddess from ancient Mesopotamian mythology which is substantially different (and did not have multiple heads).

Tiamat is a queen of evil dragons and a member of the Untheric pantheon of FR gods.

Like most other draconic deities, she is the offspring of the dragon creator deity Io. She is the eternal rival of her brother Bahamut, the ruler of the good metallic dragons. She dwells in Avernus, the first layer of the Outer Plane of Baator (also known as the Nine Hells). The first edition of Advanced Dungeons & Dragons named her as the ruler of Avernus; later editions reserved the rulership of the layers of Baator for powerful baatezu.

Description
Tiamat has one head for each primary color of chromatic dragon (black, blue, green, red, white), and each head has the powers of a member of the respective race of dragonkind.

Tiamat was also one of the first deities to have Aspects, or lesser avatars. These Aspects may appear as powerful versions of her chromatic children or as versions of her own five-headed form.

Forgotten Realms
Tiamat is one of the few surviving gods of the Untheric pantheon (based on the Sumerian mythology), having battled Marduk in ages past, and also a member of the draconic pantheon, daughter of Io, the slayer of Gilgeam the God-king of Unther, Nemesis of the Gods.