Tiamat

Tiamat is a queen and mother of evil dragons, while good (metallic) dragons acknowledge her as being the first of the dragon kin, they don't worship her. Her symbol is a five-headed dragon.
Tiamat is the five-headed queen of the evil chromatic dragons. She has one head for each customary color of chromatic dragon (black, blue, green, red, white), and each head has the powers of a member of the respective race of dragonkind. Her body is a blending of various chromatic dragon forms with an appropriately multicolored hide. Her body also has traits in common with a wyvern, including a long tail tipped with a poisonous stinger.
Tiamat has also been known to manifest as a dark-haired human sorceress when making contact with mortals

The Banishment of Tiamat

Many myths claim that Tiamat lived for a long time on the Prime Material Plane, seeding it with evil dragons and dark magic. Eventually she was banished to The Nine Hells by all seven of The Mothers - it was one of the rare times when all of the Six and their exiled sister, The Raven Queen, came together to defeat the evil of Tiamat and banished here forever into the hells. Sealing away the different parts of her broken body, to keep her divine soul imprisoned, forever.
The parts were known as: • The Horn of Tiamat

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In the possession of Granny Nightshade who wants to free Tiamat for even more power

• The Bite of Tiamat

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On a necklace worn by Queen Empress Aadhya Prima - also on a crusade to free her dragon goddess and become her ultimate champion

• The Gash of Tiamat

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Hidden away in the depths of the frozen city of Ythryn beneath The Icefell Peaks

It is said that these parts were strewn across Revelyá, to forever keep them out of reach of the Dragon-Queen who was bound to a different plane of existence.

Beliefs & Worshippers

Tiamat is a greedy, vain, and arrogant goddess who embodies all the strengths of evil dragonkind, and few of their weaknesses. Tiamat is most concerned with spreading evil, defeating good, and propagating chromatic dragons. She never forgives a slight. Although she is not averse to razing the occasional village, her true schemes are subtle and hard to detect. From the shadows she moves her puppets' strings.
The Queen of Evil Dragons demands reverence, homage, supplication, and tribute from her subjects.
Few humans or other humanoids worship Tiamat, but her children, the chromatic dragons, all acknowledge her sovereignty. Blue and green dragons obey her most readily. Naturally, the grotesque reptilian creatures known as the spawn of Tiamat worship her as their mother. Kobolds may also revere her as their progenitor.

Lair

She dwells in Avernus, the first layer of The Nine Hells Tiamat's realm, known as the Dragonspawn Pits of Azharul or simply as Tiamat's lair, sprawls in a cluster of tall hills and mountains near a pillar made from the tormented heads of liars and a pit of maggots from which lemures emerge. One must fly (or, heavens forbid, swim) across the maggot pit to reach Tiamat's caves.
Tiamat's lair contains the main gate to the second of The Nine Hells, Dis. To reach it, one must pass a chamber known as the Cave of Greed, which is filled with cursed treasure that compels the weak-willed to try to steal it. Tiamat has her own chamber within the complex, as do each of her five consorts, but it is possible to travel to Dis without disturbing her.

Date of Birth

Unknown - predates back to the Primal Age