Tiamat

"Tiamat isn't like anything the Greek or Roman gods have ever faced before, if they succeed in bringing her back then you're doomed."

- Ishtar Tiamat is the primordial goddess of creation and chaos in Mesopotamian mythology, she is probably the oldest deity in any pantheon anywhere. Tiamat can take the form of a beautiful woman or a giant fearsome dragon. In her Dragon Form she is about twice as powerful as Typhon.

History
Tiamat the mother of the Sumerian pantheon ruled peacefully beside her husband Abzu and they shared between them the 7 Tablets of Destiny, until a trio of younger gods named Anu, Enki, and Enlil killed Abzu and took his crown and 3 tablets declaring that they now in charge of the universe. This started a war amongst the gods where the younger gods supported the three usurpers while the elder gods supported Tiamat in her campaign to avenge Abzu's death.

The rebel gods weren't as powerful but they had greater numbers, and were strengthened by the support and worship of humanity. To rectify this Tiamat brought forth monsters and demons including some dragons to aid her cause. Unlike the Titanomachy which lasted for only a decade this war lasted for centuries and as the war continued the once peaceful and benevolent Tiamat become increasingly chaotic and insane and cruel.

In the final battle of this war Tiamat was defeated her in her dragon form by shooting an arrow into her mouth to pierce her heart, afterwards Tiamat's body was cut into pieces and the gods thew half of the pieces into outer space and buried half of them in parts of the earth all over Mesopotamia.

Unlike the defeat of Kronos by Zeus, the defeat of Tiamat didn't bring stability to the Mesopotamian pantheon. After defeating Tiamat and her general Kringi, Marduk took the 4 Tablets of Destiny in their possession and decided to take his army back home and usurp the three original usurpers and take the Tablets in their possession. Marduk cut Anu and Enlil into a thousand pieces and scattered them to the underworld, Marduk spared his father Enki however.

Countdown to Tiamat's return
The Apotheosis Initiative has sent teams to excavate sites all over the middle east and sent explorers into outer space searching for the pieces of Tiamat's body so that they can help her reform. The monsters that were part of Tiamat's army during her war are now loyal to the Apotheosis Initiative.

Personality
Tiamat is insane but not in the way you might assume. Strangely for a goddess of chaos, when Tiamat manifests in dreams she speaks politely in a cheery voice, there is a disconnect between her warm sunny demeanor and her goal of genocide against the gods. She appears to rather s. Since most of the Mesopotamian gods have faded away she won't be able to satisfy her desire for revenge. Instead she now wants to destroy all the gods from every pantheon.

Tiamat is perfectly aware of the fact that once the Apotheosis Initiative doesn't need her anymore they'll try to destroy her but she says she doesn't care. If Tiamat is pushed too far her warm sunny demeanor will be replaced by raw violent bestial behavior. This is accompanied by her shifting from humanoid to dragon form.