Bahamut Replace Doom Gaze/DeathGaze?

As Tenkraider is stating if you wanted to have an event occur before the battle it means you will need to modify more things in game, not the least of which will be making a custom event.

If you only have a few lines of text you want to use I would propose using the in battle text as soon as the fight begins. You won't be able to do much, but it might do essentially what you want.

If you don't mind telling us exactly what you want to achieve and how you wish to achieve it we can probably help you in making this happen. If you are looking to make a Bahamut fight more like FF5 your best option might even be to remove Doom Gaze (or just make him drop a different item/esper) and set up a custom event somewhere else (like on the ground) that can trigger the battle.