Quote Originally Posted by Wolf Kanno View Post
Yeah it's only a mission failure if all your active party gets killed off. You basically select a mission and choose the party you intend to send and if they all get killed the game is over. Summons are pretty overpowered but summoning one sacrifices the use of a character for the rest of the mission so you're potentially weakening your team for a limited power boost, but some situations can call for needing the extra power.

The Bahamut scene you saw I believe is Type-0's Teraflare battle animation where Bahamut summons an army of lesser Bahamut's to all perform TeraFlare. Other summons to watch out for are Odin and Diablo. Summons have a story purpose in this game which is connected to the gameplay mechanic of wiping out a character who summons them.
Ok, thanks for your explanation Wolf Kanno