The Nirvana is Yuna’s Celestial Weapon and is unlocked through the Monster Arena. Once you have unlocked the Monster Arena by capturing at least one of every fiend in the Calm Lands, head back to the arena and you will see a chest on the ground. With the Celestial Mirror in hand, you will be able to open the chest for the Nirvana.
Fully powered up, the Nirvana’s abilities are as follows:
- Break Damage Limit
- Triple Overdrive
- Double AP
- One MP Cost
The Moon Crest is the first piece of a Celestial Weapon you can obtain in the game, as it’s pretty much right at the beginning when you get to Spira. On the beach where you first met Wakka, head into the water and go southeast. You should notice that you can go around the one wall there, so do that to arrive in an alcove with a chest. That chest will contain the Moon Crest.
Thankfully, you can go back and get this if you miss it the first time around. That likely won’t be until late into the main story, since that is when you gain access to the airship. Remember that this is the dock that you use to go to Kilika, so you won’t have to actually enter Besaid Village, which gets blocked by a powerful optional boss that requires very high stats on your characters.
The Moon Sigil will require a little bit of extra work on your end, since you will need to obtain all Aeons. You will get the majority of Aeons during the main story, but there are three of them that are optional: Yojimbo, Anima, and the Magus Sisters. Yojimbo can be gotten from the Cavern of the Stolen Fayth, which will require you to pay a hefty sum in order to him to join you. Anima requires you to complete all six Temples and secure the treasures from the Destruction Spheres. Once you do this, visit Baaj Temple via the airship, defeat Geosgaeno, and enter the temple underwater to get Anima.
For the Magus Sisters, you will need the two Aeons above, then visit Remiem Temple via the Calm Lands. Inside the actual temple, you will find Belgemine, who will continue your “training” with Yuna’s Aeons. Defeat her Aeons, up to and including Bahamut, to receive the Flower Scepter. Once you have that and the Blossom Crown (given to you by the Monster Arena NPC from capturing at least one of every fiend on Mt. Gagazet), head to the back of the room with Belgemine to break the seal and receive the Magus Sisters. Finish the rest of the battles with Belgemine, with her Magus Sisters being last, then choose to send her to get the Moon Sigil.