Remiem Temple holds two sidequests for the game, one for Chocobo Racing and the other involves the resident Aeon expert, Belgemine. In order to get to Remiem Temple, you need to have completed the Chocobo Training course, Wobbly Chocobo. This allows you to ride chocobos in the Calm Lands, so grab one from the trainer and head back up to the entrance. To the right of the entrance, you can find a feather on the ground, so interact with it while riding a chocobo to jump over the gap and find the entrance to Remiem Temple. The actual inside of the temple is where you will meet up with Belgemine again, who is going to test Yuna’s strength as a summoner.
You will have to fight through all of the Aeons here, starting with Valefor, although she will only let you fight the ones you have. So, if you don’t have Yojimbo or [Anima] yet, then you will want to go get them right now. How easy or hard things are depends solely on how well you built Yuna and when you do these fights. If you’re doing these at the end of the game, as you’re building the characters in anticipation for the Monster Arena and Dark Aeons, then you will have an absolutely easy time. If you are having trouble at all, your best bet is to just work on building up Yuna. If you happen to do the [AP Grinding] trick and fill out the entire Sphere Grid for Yuna without putting in any extra spheres, you should be more than powerful enough to easily dispatch each Aeon battle.
The battles you will face are as follows:
Your reward for defeating Bahamut is the Flower Scepter, one of the two items needed to unlock the Magus Sisters. The other item, the Blossom Crown, is given to you by the Monster Arena NPC for capturing at least one of every fiend on Mount Gagazet. Once you have both of these items, as well as Anima and Yojimbo, approach the Chamber of the Fayth door in the back of the temple and offer both items. You will be able to acquire the Magus Sisters then, which will allow you to face off against Belgemine’s version. Defeat her Magus Sisters and you will get an option to Send her or not. Do it and you will receive the Moon Sigil, one of the pieces for Yuna’s Celestial Weapon. Note that if you do send her, Belgemine will disappear entirely and you will not be able to challenge her Aeons anymore.