Tiamat is a gigantic dragon, representing Chaos and the saltwater ocean. In union with Apsu, freshwater (or sweet water), she became the mother of the gods, and existence.
According to the epic poem Enuma elish, which dates back to about 1900 BCE, the decendants of Tiamat and Apsu became irritating, so Tiamat proposed that they kill their own offspring. Ea discovered these plans and killed Apsu while he was sleeping.
Tiamat took a new consort, her son Kingu, and created an army of monsters to avenge the death of Apsu. Marduk led the gods against the new pair and defeated Tiamat, cutting her body in half. From the upper half, Marduk created the sky, and from the lower half, the earth. From Kingu's blood, Marduk created the first humans.