Tiamat in this game borrows heavily from the celtic Morrigan, the Indian Kali-ma, the sumerian Tiamat, Morganna le Fey in Arthurian legend, and even Jason/Medea.
Sumerian-Babalonian Creation Myth
Ok, well, let's see what we can piece together. There is some very interesting information on Tiamat to be found in Babalonian and Sumerian creation myth. There is an interesting tale, the Babalonian "Epic of Creation--Enuma Elish" which was written around the 12th century BCE. See what has been translated from the tablets here.
http://www.dreamscape.com/morgana/babylon.htm
Certain aspects of these tablets sound very familiar. This is one passage as an example.
They are banded together and at the side of Tiamat they advance;
They are furious, they devise mischief without resting night and day.
They prepare for battle, fuming and raging;
They have joined their forces and are making war.
Ummu-Hubur, who formed all things,
Hath made in addition weapons invincible; she hath spawned monster-serpents,
Sharp of tooth and merciless of fang.
With poison, instead of blood, she hath filled their bodies.
Fierce monster-vipers she hath clothed with terror,
With splendor she hath decked them, she hath made them of lofty stature.
Whoever beboldeth them, terror overcometh him,
Their bodies rear up and none can withstand their attack.
She hath set up vipers, and dragons, and the monster Lahamu,
And hurricanes, and raging hounds, and scorpion-men,
And mighty tempests, and fish-men, and rams;
They bear merciless weapons, without fear of the fight.
Her commands are mighty; none can resist them;
After this fashion, huge of stature, hath she made eleven monsters.
Among the gods who are her sons, inasmuch as he hath given her support,
She hath exalted Kingu; in their midst she hath raised him to power.
To march before the forces, to lead the host,
To give the battle-signal, to advance to the attack, To direct the battle, to control the fight,
Unto him hath she entrusted; in costly raiment she hath made him sit, saving:
I have uttered thy spell; in the assembly of the gods I have raised thee to power,
The dominion over all the gods have I entrusted unto thee.
Be thou exalted, thou my chosen spouse,
May they magnify thy name over all of them...the Anunnaki.
She hath given him the Tablets of Destiny on his breast she laid them, saving:
Thy command shall not be without avail, and the word of thy mouth shall be established.
Now Kingu, thus exalted, having received the power of Anu,
Decreed the fate for the gods, her sons, saying:
"Let the opening of your mouth quench the Fire-god;
Whoso is exalted in the battle, let him display his might!"
A somewhat similar summarized version (for those who don't read) is below.
In Babylonian myths, Tiamat is a huge, bloated female dragon that personifies the saltwater ocean, the water of Chaos. She is also the primordial mother of all that exists, including the gods themselves. Her consort is Apsu, the personification of the freshwater abyss that lies beneath the Earth. From their union, saltwater with freshwater, the first pair of gods were born. They are Lachmu and Lachamu, parents of Ansar and Kisar, grandparents of Anu and Ea.
In the creation epic Enuma elish, written around 2000 BCE, their descendants started to irritate Tiamat and Apsu so they decided to kill their offspring. Ea discovered their plans and he managed to kill Apsu while the latter was asleep. Tiamat flew into a rage when she learned about Apsu's death and wanted to avenge her husband. She created an army of monstrous creatures, which was to be led by her new consort Kingu, who is also her son. Eventually, Tiamat was defeated by the young god Marduk, who was born in the deep freshwater sea.
Marduk cleaved her body in half, and from the upper half he created the sky and from the lower half he made the earth. From her water came forth the clouds and her tears became the source of the Tigris and the Euphratus. Kingu also perished, and from his blood Marduk created the first humans.