Tiamat

Sister of the Platinum Dragon, this 5-headed chromatic entity had a more difficult time of integrating with the beings who populated the material plane, and her brother quickly grew more popular, while growing anti-dragon sentiments pushed the people farther and farther away from her. Where Bahamut was considered gracious and beautiful, she was seen as a monstrosity, and she retreated to her own realm in the hells to be as far away from her brother and father as possible.
During her quest to establish her own realm, Tiamat's green head was severed from the rest of her being and lost on the material plane. A cult formed around this creature, believing it to be a serpent goddess named Teyamut. Eventually, separated from the rest of the divine being of Tiamat and trapped on the decidely not divine material plane, the green head died, but the cult conitnued. They fought outsiders who claimed they worshipped a piece of an ancient dragon god, but secretly, members of the cult's inner circle plotted to return the head to the archdragon.
In year 203 of the Age of Dragons, Rassha Arboc, on behalf of the Cult of Teyamut, infiltrated an expedition to reunite Em'jaygh with his father. The cult killed Em'jaygh and fused his soul with the skelton of Teyamut, reuniting Tiamt with her lost fifth head and permanently weakening Bahamut by robbing him of his son.