Tiamat

Tiamat is the goddess of Chromatic Dragons. Like her counterpart Bahamut, she was a Titan first called into being as a heavenly mount for the gods, but broke through her reins and became too powerful for even the gods to control.
Her avatar is that of an enormous five-headed Dragon, with each head corresponding to one type of Chromatic Dragon (red, blue, black, green, and white, in that order). A single destructive breath from any of her heads is enough to obliterate an army in one fell swoop.
Tiamat was banished to the caves of Avernus, on the first level of the Nine Hells, in the Age of Audacity. Projections and avatars of her power have often fought in the Blood Wars and the age-defining wars between the gods, but her primary presence remains in the Hells.