Tiamat is a green dragon with a morphology similar to Nidhogg. Her most distinguishing traits are the spiny frills on her head, along with a beak-like mouth. Crystalline growths can be seen on the inside and underside of her jaw, as well as from the various piercing wounds on her body, caused by the neurolink pins keeping her in place.
During the Third Astral Era Bahamut fell to the invasion of the Allagan Empire under the reborn Emperor Xande. In her grief, Tiamat was manipulated by the Ascians into summoning him back as a Primal—something she quickly regretted as the Dreadwyrm was nothing but a crude imitation of the original Bahamut. The Ascians aided the Allagans in capturing the primal Bahamut and entombed him within Dalamud where he remained until Nael van Darnus summoned him back at the Seventh Umbral Calamity.
Tiamat and many other Meracydians were captured by the Allagans and taken to Azys Lla where they were confined to the Delta Quadrant. Regretting the mistake she made with the Ascians, Tiamat made no effort to break from her bindings in penance, even long after Allag fell.
When the adventurer arrives at the Delta Quadrant, Midgardsormr explains that long before Ishgard's base betrayal, another group of mortals sought to enslave his children, with repercussions to this very day. He asks that they meet someone here.
At the Antithesis they find the imprisoned Tiamat, who explains her story and how the "dark ones" were manipulating them long ago. She warns that Allag sealed away many primals long ago, and explains the true key to peace is to eliminate the Ascian threat. When Midgardsormr asks if she will finally forgive herself and break free of her bindings, she states she does not wish to do so after the terrible mistake she made and remains penitent out of guilt.
Tiamat is a primordial goddess of the ocean in Mesopotamian religion. The game Dungeons & Dragons has Tiamat as a chromatic dragon, the Queen of the Evil Dragons, and as such the opposing deity to Bahamut, who is a platinum dragon and the King of Benevolent Dragons.