Tiamat the many-headed and many-hued was a deity in the TDDC universe. Of the three goddesses who donated a portion of their divine power to a mortal race (the others being Blibaal and Lolth), she gave by far the most, causing her chosen people (the dragons) to themselves be almost godlike in power, as evidenced by their (disputed) creation of the Dragonborn.
However, her greed was her undoing. Seeing the sun under Pelor's control, she became so taken with it that she begged Vecna to tell her how she could take it. In exchange for some unknown boon, he told her the secret of The Death Equation, which she used to slay Pelor and steal the sun. However, this weakened her so much that Delvash was able to capture her, torture the secret of The Death Equation out of her and eventually use it on her, ending her otherwise immortal life.