FFX - Sidequests - Remiem Temple
There is a hidden temple in the mountains to the east of the Calm Lands. There you can battle against a fellow summoner named Belgemine. She has the exact aeons you do. The first few aeons you defeat will get rare items. Defeat her Bahamut though and she will give the Flower Scepter. This is a key item in acquiring the Magus Sisters. Go to the Aeons section for more detailed info on obtaining this powerful summon. If you defeat all her aeons including the Magus Sisters she will give you the Moon Sigil. This is used to power-up Yuna's legendary weapon.The key item Cloudy Mirror can also be acquired here. You can do this by winning the chocobo race underneath the Remiem Temple. Winning the race is very easy even on the first try. For a better time try to get as many shortcuts and avoid as many poles as possible.