The Monster Arena is a huge sidequest located in the Calm Lands, where the player is tasked with capturing monsters in the game. This is done by defeating them with weapons that contain the Capture ability on it (something that can only be bought from the Monster Arena itself).
As you capture the fiends from the various areas, you will be able to fight them at any time. The first time you battle them is free, but any subsequent times will cost you a fixed amount of gil. Fulfilling specific conditions (such as 3 of each wolf in the game) will unlock unique fiends that cannot be fought elsewhere.
