Dark Bahamut is one of the Dark Aeons and spawns after you have obtained the airship Fahrenheit. Dark Bahamut can be found in the Zanarkand Ruins and blocks the path that leads to the 
Sun Crest.
Travel back to the Zanarkand Ruins. Use the teleportation pads to head back to the Zanarkand Cloister of Trials and take the elevator down to the lower level. Head in to the Chamber of the Fayth and then in to the back room (called “The Beyond”) where you fought Yunalesca previously.
Each of the Dark Aeons, including Dark Bahamut, are considered superbosses and are some of the strongest enemies that you can fight in the game. As such, you will need to ensure that your party is fully leveled, has their best weapons and has their stats maximized as much as possible. Refer to the Advanced area of the Tips and Tricks section for additional details.
Also note that Dark Bahamut has extremely high Evasion which means that it will dodge most of your physical attacks. This can be counteracted by ensuring that your characters have a very high Luck stat, but grinding 
Luck Spheres and 
Fortune Spheres can be a painstakingly long process. High Luck is not required but will make the battle much easier.
Preparing for battle against Dark Bahamut involves a few extra steps. Make sure that you have at party members that are equipped with the 
Stoneproof or the Ribbon auto-abilities added through customization. You will also need to have Auto-Phoenix and Auto-Protect equipped. Your party members will likely die several times during the course of the battle.
HP: 4,000,000
Elemental Affinities: None
Immune to Sensor
Dark Bahamut has two attacks that will prove difficult to deal with. The first of these is his Overdrive, Mega Flare, which will hit your party members for over 40,000 damage. The strategy for dealing with this is to have 
Auto-Life cast on each of your party members so that they may immediately recover as soon as Mega Flare has been cast.
It will also use an attack called Impulse after every 5 attacks that you inflict (including ones that miss). The challenge with this fight is ensuring that you have enough time to recover in between Mega Flare and Impulse attacks. Do not attack Dark Bahamut if a Mega Flare attack will be following shortly thereafter.
You can Steal the following items during the battle:
Twin Stars (x2) (common)
Elixir (rare)
And Dark Bahamut will drop the following items when it is defeated:
Dark Matter (common)
Master Sphere (rare)
