Tiamat according to the Babylonian Epic of Creation that was written in the first millennium B.C.E, is an ancient sea goddess who gave birth to all. She was part cosmic serpent, part winged animal, her image may appear more like a dragon than demon. Within her she carries the essential DNA of all demonic species: the dark, creative, turbulent, protean spirit of the unconscious deep.

"When skies above were not yet named
Nor earth below pronounced by name
There was water..."

And Tiamat mingled her salt seas with the fresh waters of Apsu, her consort, and bore populations of gods who lived within her darkness until finally Apsu could no longer bear the disorder and clamor of the young gods. He attempted to destroy their offspring. Naturally enraged, Tiamat collaborated with her son and destroyed Apsu. Generations passed until her great-great grandson, the solar god Marduk, challenged her dominion.

~ A Field Guide to Demons, by Carol and Dinah Mack.

The above story much resembles that of the creation of the Greek Gods, when Zeus killed his own father.
The following Epic of Creation was read annually at the Babylonian new year's festival. Since it features the slaying of Tiamat by Marduk, the supreme god of the Babylonian pantheon, the story is naturally told from his point of view. The ancient goddess is seen as demonic in the eyes of the new hegemony: male sun god defeats dark feminine life force of chaos and creates civilization.

Marduk was the perfect hero, having four eyes and four ears and could breathe fire. In preparation for the battle Marduk made a bow and arrow and a huge net. Carrying a spell on his lips, an herb in one hand that worked against Tiamat's poisons, and a mace in the other, he mounted his terrifying storm chariot and marshaled the seven winds to follow him into battle.

Tiamat was infuriated, her rage brought forth monsters, demons, horned snakes, bull men, fish men, filled not with blood but venom. Her army was radiant and terrible. She appointed Kingu, a monster offspring, to be her spouse and to lead her brood into battle. But Marduk challenged her to single combat. He caught her in his net and then sent evil winds toward her. She opened her mouth like a mammoth cave to swallow them, but the winds were so powerful her jaws were forced to remain open. The winds distended her belly. Marduk entered and saw her an entire army of gods, snakes, and demons. He shot his arrow that split her heart in two. As she was perishing, he stood on her body and smashed her skull with his mace.

Then Marduk sliced Tiamat in two like a cosmic clam, and raised one half of her to become the roof of the sky. He bolted it to hold the waters in check. With her lower half, he created the earth above the subterranean waters. From her eyes he created two rivers; from her udder, mountains and foothills. From her saliva he made rain and clouds; from her poison, fog. After Marduk named each thing and set the stars and gods in their places, he created man out of the blood of Kingu, poisonous spouse-creation of Tiamat.

Even though Marduk is the supreme god of the Babylonians, they cannot forget Tiamat. Nothing could have beeen created without her essence. Tiamat is primordial chaos. Homo sapiens can only walk about and build civilizations in an ordered universe, and so Tiamat was divided and named, but within and of Tiamat is all life. From this inchoate broth comes tides, fish, birds, flowers, weeks, night and day.