Nirvana is a Celestial Weapon and it is the most powerful weapon for Yuna in the game.
You must obtain the Celestial Mirror before you can obtain or create any of the Celestial Weapons. Click on the link below for more information on how to obtain the Celestial Mirror:
In order to create the fully powered-up Nirvana you will need to obtain the Nirvava weapon itself first, then use the 
Moon Crest and the Moon Sigil to power it up.
You can obtain the weapon by visiting the Monster Arena in the Calm Lands. The Monster Arena is located on the east side of the Calm Lands.
Speak to the man in front of the tent and he will ask you to capture one of each of the monsters in the Calm Lands using the weapons that he sells to you. You will need to capture one of each of the following:
Return to the man at the Monster Arena and a treasure chest will appear in front of him. Use the Celestial Mirror to open the chest.
The Moon Crest can be found in a treasure chest on Besaid Island. The treasure chest can be found just off to the right of where Tidus first arrives out in the water. Use the screenshots below to help find it.
You will receive the Moon Sigil after you defeat Belgemine at Remiem Temple. You will need to defeat each of her Aeons including the 3 optional / secret Aeons. Check out the Remiem Temple section for additional details:
Return to Macalania Woods to the area where you received the Celestial Mirror once you have obtained both the Moon Crest and the Moon Sigil. Offer Nirvana up twice to fully power it up using both the Crest and Sigil.
Nirvana has the following abilities when fully powered up:
Break Damage Limit - Allows Yuna to do more than 9,999 damage in a single attack
Triple Overdrive - Charges Yuna’s Overdrive at triple the speed
Double AP - Allows Yuna to earn double the AP for each battle
One MP Cost - All of Yuna’s magic spells will cost 1 MP
