Tiamat

In the religion of ancient Babylon, Tiamat is a primordial goddess of the salt sea, mating with Abzû, the god of fresh water, to produce younger gods. She is the symbol of the chaos of primordial creation. She is referred to as a woman, and described as the glistening one. It is suggested that there are two parts to the Tiamat mythos, the first in which Tiamat is a creator goddess, through a sacred marriage between salt and fresh water, peacefully creating the cosmos through successive generations. In the second Chaoskampf Tiamat is considered the monstrous embodiment of primordial chaos. Some sources identify her with images of a sea serpent or dragon.

In the Enûma Elish, the Babylonian epic of creation, she gives birth to the first generation of deities; her husband, Apsu, correctly assuming they are planning to kill him and usurp his throne, later makes war upon them and is killed. Enraged, she, too, wars upon her husband's murderers, taking on the form of a massive sea dragon, she is then slain by Enki's son, the storm-god Marduk, but not before she had brought forth the monsters of the Mesopotamian pantheon, including the first dragons, whose bodies she filled with "poison instead of blood". Marduk then forms heavens and the earth from her divided body.

Tiamat in The Case of the Toxic Spell Dump
There had been a cult that worshiped Tiamat in the past, but it did not survive into modern times, thus the goddess' Power disappeared from the world. David Fisher was somewhat relieved by this, as an angry Tiamat would have been an unstoppable opponent.