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. The Ascians manipulated the grieving Tiamat 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.
The Allagans captured Tiamat and many other Meracydians and took them 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. They find the imprisoned Tiamat at the Antithesis 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.
The Warrior of Light, Estinien Wyrmblood, G'raha Tia, Alphinaud and Alisaie Leveilleur visit Tiamat for information about Lunar Bahamut. Tiamat says she did not summon him, before realizing that Meracydian dragons tempered by the Telophoroi have done it. Discovering that Tiamat herself was mildly tempered upon summoning the primal Bahamut, the group convinces her to join their cause while curing her tempering. Alphinaud releases her restraints; now free from Bahamut's lingering influence and determined to atone for past sins when it means killing those among her children that have mutated beyond restoration by the Allagans' actions, Tiamat flies to Paglth'an with Estinien in tow to assist the Eorzean Alliance. Tiamat and Estinien engage Lunar Bahamut and ground the primal for the Warrior to finish off. Tiamat thanks the Scions of the Seventh Dawn, pledging that her remaining children will never again be deceived by the Ascians before flying away.
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 Umbral Calamity and besmirch his memory. She refuses to be released, having resigned herself to her fate until the realm's end. However, it is later revealed that she had been tempered by Bahamut. Despite being under his influence, her strong will allowed her to retain her sense of self and keeps herself locked away in fear of hurting innocent people. After being cured of her enthrallment, her fears have all but vanished and moved by Estinien's words, vows to protect her children.
Tiamat appears as an ally NPC in the dungeon Paglth'an. She assists the party throughout most of the instance, using attacks borrowed from Nidhogg to deal heavy damage to enemy mobs.
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.