Final Fantasy X/Secrets and Sidequests/Remiem Temple

Remiem Temple is a hidden area in the Calm Lands, available as soon as you have trained a chocobo. To train a chocobo, approach the trainer near the cliff (look at the Chocobo Trainer section for more information). Once you have the chocobo, go up towards the entrance of Macalania Woods. Here, on the east from the entrance, is a Chocobo Feather. After approaching this and pressing X, you will jump to a hill. Enter the area up ahead. You are know at Remiem Temple.

Belgemine
The summoner Belgemine will appear in Remiem Temple after defeating her at the Calm Lands. Here, Yuna and her can have an Aeon duel. In order to unlock the Magus Sisters, you must defeat all the Aeons, including the hidden Aeons, Yojimbo and Anima. To get Yuna's Moon Sigil, you must also defeat the Magus Sisters and then send Belgemine.

Aeon Battles
Below are strategies on defeating Belgemine's Aeons.

BOSS BATTLE: Valefor
HP: 20,000

Valefor is the easiest Aeon. If you can't beat it, you should probably train Yuna a little more. If you have the secret Aeons at this point, Valefor can be killed in an instant. Otherwise summon Ixion or Shiva (Ifrit's a little too slow for Valefor, although it is possible) and keep up a constant stream of attacks. If you are injured to greatly, heal the Aeon with their own Black Magic. If Valefor goes in to Overdrive, use shield. Valefor will attack with Energy Ray and then, afterward, Energy Blast, so your Aeons may not be able to withstand. If that is the case, summon another elemental Aeon. If you want to defeat Valefor quickly, summon your Aeons and boost them in to Overdrive. Two Overdrives and another strong attack should give you victory.

BOSS BATTLE: Ifrit
HP: 28,000

Ifrit is easy enough. If you've got the secret aeons, just summon and pound away with Blizzaga until it dies. If you don't have the secret aeons, you could substitute Bahamut for Anima (although it wouldn't work as good because of Bahamut's lower stats). Or, you could just summon Shiva, healing yourself and hurting Ifrit with Blizzard, and wethering yourself in to Overdrive.