The Remiem Temple is a hidden temple to the southeast of the Calm Lands. It can only be reached by riding a Chocobo and interacting with a yellow feather at the southeast corner of the Calm Lands, near the southern entrance.
A Save Sphere can be found near the temple’s doors. Before going inside, take the path to the left to find the Al Bhed Primer vol. XXIV.
Interact with the Chocobo on the right side of the temple to start a Chocobo racing mini-game. Reach the bottom before the champion to obtain the Cloudy Mirror. This item is necessary to open locked treasure chests around Spira. You can easily win by not opening any treasure chests. After winning the item try the mini-game again, but this time try opening five treasure chests and still win against the Chocobo Knight to earn a trophy.
When you’re done with Chocobo racing, enter the temple to find Belgemine. Speak with her to battle her Aeons yet again. Defeat all Aeons to receive the Flower Scepter, which unlocks one of the doors in the back where you can receive your final Aeon. If you came here the first time you visited the Calm Lands, you will need to return later after obtaining the other Aeons – Yojimbo and Anima.
To unlock the other door, you must capture all types of fiends on Mt. Gagazet and speak with the man at the Monster Arena in the Calm Lands. He will give you the Blossom Crown, which unlocks the final door and allows you to receive The Magus Sisters.
Defeat Belgemine’s Magus Sisters to receive Yuna’s Moon Sigil.