Tiamat is the Queen of the Chromatic Dragons and lives within the Depths of Hell. As such, she is in league with Asmodeus and rules over the largest district of hell. The Queen of Dragons was once married to Bahamut, but her crusade against non-draconic kind has placed her in a feud with her husband. She had attempted to fight the non-draconic denizens of the world by way of the Feywild, but Bahamut prevented her ascendance and sealed her in the Depths of Hell.
Her followers look to break the chains that bind her to the Depths of Hell, but have come up with nothing. Only the power of Bahamut can release her from her bonds.