Tiamat

Tiamat was the lawful evil dragon goddess of greed, queen of evil dragons and, for a time, reluctant servant of the greater gods Bane and later Asmodeus. Before entering the Faerûnian pantheon, she was a member of the Draconic pantheon, and for some time she was also a member of the Untheric pantheon.

Tiamat was also the eternal rival of her brother Bahamut, ruler of the good metallic dragons.