Tiamat is the Goddess of Evil Dragons and currently resides in The Nine Hells.
Like her brother, Bahamut, she too was once a mortal dragon-born, though through her own treachery and perseverance, she rose to commander of armies and eventually status of God.
Tiamat was born during the same cycle as her brother, in roughly 710 B.B. She, unlike her brother, stayed in The Garden City of Tefalls. There she fell in to a bad crowd of murderers and thieves and learnt most of her tricks and skills as a rogue.
Over the time of her brother's training as a Paladin, Tiamat rose through the ranks of the criminal world and eventually was able to build her own army to take Cawic The City of Bells for her family name, her brother also marching in a bid to stop her in a clash later known as "The Battle of Dragon's End".
Whilst the two armies raged in the City of Bells, the two leader's battle took place at a site just north of the city. (A site that would later become The Temple of Elemental Evil.) There, Bahamut slew his sister and ascended to godhood. Tiamat's soul was lost in the Domains, barred by Pelor to enter Arborea as a spirit.
She wandered The Astral Plane until eventually arriving in The Spider Pits of Lolth. There, the evil Goddess used Tiamat as a test subject to create her own Goddess. Tiamat was raised as a Goddess of Evil Dragons, though her half-human side was destroyed in the process, causing her to change into a fully formed dragon with five heads.
After ascending, Tiamat roamed the Astral Plane searching for Arborea to exact her revenge upon Pelor and her brother. There she came across Asmodeus, whom used his sorcery to chain her power to his soul. When Asmodeus fell, causing the Nine Hells to be created, Tiamat was trapped in the seventh layer. She named this place her new domain and resides there permanently until her followers can find a means of freeing her.