The Ultima Weapon is the strongest keyblade in the game and to obtain it requires obtaining many materials. Once you have completed the story you will make this powerful blade in preparation for the fight against Dark Inferno.
The Ultima Weapon has 13 Strength, 13 Magic and unlocks the abilities Combo Boost, Air Combo Boost and Situation Boost. Once this weapon transforms it is able to deal a huge amount of damage, more than any other keyblade.
You will need to gather 58 different Synthetic Materials. These can be obtained by fighting enemies, after boss fights, or in the chests that are scattered throughout the game. Once you have the required number of Synthetic Materials the Ultima Weapon recipe will become available at any Moogle Shop.
You will now need to obtain 7 Oricalums+, listed below in no particular order is how you can get them.
• Photograph all 90 Lucky Emblems.
• Defeat the Eclipse Galaxy Bosses.
• Get the high score in all Flantastic Seven minigames.
• Get all the treasure in the sled minigame in the Frozen's world.
• Reach the chest in Caribbean World hidden on one of the islands.
• Reach the chest in the Keyblade Graveyard in the End of the World.
• Buy items in the Moogle Shop and exchange Prize Postcards for rewards in the mailboxes - one of the rewards is Orichalcum+.
And finally you need 2 Wellspring Gems, 2 Pulsing Crystals, and 2 Lucid Crystals. Now with the recipe and the required ingredients you should be able to craft Ultima Weapon.