Tiamat is an optional boss in Final Fantasy VIII, encountered in the Clocktower of Ultimecia Castle in the game's final stages. It is arguably the strongest of Ultimecia's eight minions. Tiamat looks very similar to Bahamut and uses a similar move. The Scan description indicates that Tiamat was once a GF but was turned evil under Ultimecia's power.
During the battle, Tiamat will start to elevate higher into the air and begin counting down to its signature move: (D-A-R-K-F-L-A-R-E). This attack is an upgraded version of Bahamut's Mega Flare and deals heavy damage regardless of the characters' Spirit stat. It is a Fire-elemental attack, which means characters need to increase their Fire defense (such as junctioning Flare to Elem-Def) in order to reduce damage from Dark Flare. In fact, the battle can be made extremely easy by junctioning the characters to absorb the element, which will render Tiamat unable to kill the party, as D-A-R-K-F-L-A-R-E is its only attack.
In the game's American and European versions Eden can be drawn from Tiamat if the player did not draw the GF from Ultima Weapon earlier.