Bahamut is the fifth Aeon that Yuna can summon. She receives it after the completion of the Bevelle Cloister of Trials. Bahamut is one of the most prominent summons who appears as a character in nearly all of the Final Fantasy titles.
Overdrive: Mega Flare
Bahamut is the most powerful Aeon you receive as part of the regular story. As soon as Yuna receives it after the completion of the Bevelle Temple you will begin to rely on Bahamut fairly frequently (starting with the Via Purifico).
He can do regular attack as well as an attack called Impulse. Impulse hits all enemies but it takes more time for Bahamut to recover from Impulse from a timing perspective. Use regular attacks if there is only one enemy.
Bahamut is capable of doing more than 9,999 damage as soon as it joins you as an Aeon.