Tiamat is an ancient figure in Pulsian legend, a dragon said to work under the King of Dragons himself, Bahamut. Tiamat is said to have an affinity for water, and was at one time known as the Guardian of the Seas.

Appearance
It is not known what Tiamat actually looks like, as only old drawings and archives actually detail his existence, and of those, many are only what people thought he looked like. There have been no confirmed sightings of the mythical dragon, the so-called 'Guardian of Seas.'
What has remained constant within the Pulsian records is the general appearance and actions taken by the mighty creature. Tiamat is said to have scales the color of the ocean, so as to blend when he traveled; he is a dragon in appearance, and is said to have claws like diamonds and teeth like razors.
The symbol commonly associated with Tiamat, and which is said to be inscribed somewhere on his body, presumably on his chest or back, is a tribal sort of diamond. The symbol also appeared in relation to the Dragon King's Temple, backing up the theory that Tiamat serves, or served, Bahamut at one point.
In Pulsian Legend

Scrolls and documents unearthed from Oerba's text archive state that in ancient times, Tiamat was the partner of a renowned but cowardly man by the name of Stima, who protected a shrine dedicated to Bahamut.
Allegedly, the two of them were ambushed by an enemy battalion they had bested in battle years before, and Tiamat was gravely injured. Seeing his partner unable to fight, Shima attempted to run from the battle. He did not get the chance. Bahamut swept down from the sky to survey his guardians in battle, and upon seeing the injured state of his kin, coupled with the attempted escape of his partner, unleashed a Mega Flare, killing the man instantly.
Just as swiftly, Bahamut killed all but one of the attackers, who was nothing but a mere child by his standards, a minor, and sent him back to his village unharmed. The story goes that he took Tiamat into his arms that day and healed him, made him the guardian of his temple in the mountains, the Dragon King's Temple. It is also said that he swore no human would be allowed to guard his temple again, and no human would set foot inside without first proving themselves.
It is not said what happened after that, or how Tiamat and Shima came to be partners, but there are records that prove the existence of a set of caves in the mountains to the east of Oerba, known as Talon's Cave, which suggests it could be the location of the temple.
Trivia
- Tiamat is the name of a reoccurring summon in the Final Fantasy franchise.