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. There are crystalline growths 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.
Though Tiamat remains angry over the Allagan Empire attacking her kind and killing Bahamut, leading her to be manipulated by the Ascians, she doesn't allow it to cloud her reason like Nidhogg. She resents the Ascians for using her and causing her people so much grief and is saddened that war continues to rage between man and dragon.
Upon her imprisonment within Azys Lla, Tiamat has surrendered to her sorrow and guilt, unable to forgive herself in creating the primal that would cause the Calamity and besmirch his memory. She refuses to be released, having resigned herself to her fate until the end of the realm.
When Midgardsormr and the First Brood arrived on Hydaelyn, the dragons spread across the realm. Bahamut and Tiamat traveled to Meracydia where they created many dragons on the southern continent.
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 regretted as the Dreadwyrm was nothing but a crude imitation. The Ascians aided the Allagans in capturing the primal 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 Warrior of Light 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. At the Antithesis they find the imprisoned Tiamat who tells her story and how the "dark ones" were manipulating them long ago. She warns that Allag sealed away many primals and explains the key to peace is to eliminate the Ascian threat. Midgardsormr asks if she will finally forgive herself and break free of her bindings, but she does not wish to do so after the 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.