The Remiem Temple is an optional area within the Calm Lands, and you can access it at any time after completing the first part of the Chocobo Trainer minigame in the same area. To find the Temple, ride a chocobo in the Calm Lands, then head to the southeastern part of the map, where you will see a chocobo feather. Press when on it to jump across to a new platform. Then, move forward to enter the area.
When you enter the area, cross the bridge and head to the west side of the temple. Here you will find the Al Bhed Primer Vol. XXIV, and a sphere on the ground. Interact with the sphere to unlock the Chocobo Racing minigame — the item will explain how to begin it. Now, head to the east side of the temple and talk to the chocobo there to start the minigame.
The goal of this minigame is to get to the bottom of the track before the other chocobo does. Unlike the Chocobo Training in the Calm Lands, you will have full control of your chocobo here. Throughout the track you will see chests — if you open one, your chocobo will jump to a different part of the track. The first time you win this race, you will receive the Cloudy Mirror.
You can win other prizes depending on how many chests you open before winning the race (note that you can only get these once):
While the most important thing to get from the minigame is the Cloudy Mirror, it could be worth getting the Wings to Discovery if you’re planning on fighting the game’s superbosses: you can use them to customize a character’s armor to get the ‘Break HP Limit’ passive, which costs 30 Wings of Discovery (and this game is by far the quickest way to get 30 quickly).
If you win the race after opening 5 chests, you will earn the ‘Chocobo Master’ achievement.
When you’re done with the Chocobo Racing, you can head into the Temple.
Inside the Temple
In the Temple you will find Belgemine, as well as a sealed door. Here you are able to challenge Belgemine like you did previously in the game, but this time you will be able to fight every Aeon that you’ve unlocked so far.
However, you won’t be able to do this immediately. First, you will only be able to fight Valefor, and you will unlock the next Aeon (Ifrit) by defeating her. Defeating each Aeon in succession will unlock the next one in line. The order will match how you unlocked the Aeons throughout the game, and will include the optional Aeons as well.
Each Aeon you defeat will also give you a reward the first time you beat them:
The most notable rewards are the Flower Scepter and the Moon Sigil. The Flower Scepter is needed to break the seal on the door and obtain the Magus Sisters, while the Moon Sigil is needed to upgrade Yuna’s Celestial Weapon to its full potential.