She was created when the god Io was killed in the Dawn War. Io was cleaved in half by the primordial Erek-Hus, and when his body fell, his two halves arose as two new gods. His right half took on Io's traits of avarice, pride, and arrogance, and became Tiamat.
Tiamat appears as a massive, five-headed chromatic dragon with a long tail tipped with a poisonous stinger. Each head is a different color, representing each of the chromatic dragons. She has also been known to appear as a beautiful, but unquestionably powerful human sorceress.