Ultima Weapon

The Ultima Weapon (アルテマウェポン) is a keychain for Sora's Keyblade which appears in Kingdom Hearts, Kingdom Hearts: Chain of Memories, and Kingdom Hearts II. It is the most powerful keychain, and requires the most effort to obtain in all three games. Unlike other keychains which appear multiple times, the Ultima Weapon's appearance differs between games. In Kingdom Hearts it is the strongest Keyblade in terms of strength and MP, but in Kingdom Hearts II it is second in strength to the Fenrir and second in magic to the Fatal Crest. In Kingdom Hearts II Final Mix, it is third in magic to Winner's Proof. However, its MP Hastega ability is much more useful in normal combat than the abilities attached to these other keyblades.

Kingdom Hearts
Ultima Weapon can be synthesized once the fifth list is unlocked.
 * 5 Lucid Gem
 * 5 Power Gem
 * 5 Thunder Gem
 * 3 Mystery Goo
 * 3 Gale

Kingdom Hearts Final Mix
Ultima Weapon can be synthesized once the sixth list is unlocked.
 * 5 Thunder Gem
 * 5 Mystery Goo
 * 3 Mystery Power
 * 3 Gale
 * 3 Dark Matter

Kingdom Hearts II
The Ultima Recipe can be found at the Basement. The Ultima Weapon is a rank S synthesis item that provides 119 EXP. An Energy Crystal is required to reduce the Orichalcum+ required for synthesis, as there are only 7 in the game; this means that the Ultima Recipe cannot be synthesized unless the Moogles are at least level 2. As Serenity items are part of the recipe, they cannot be used to upgrade it.
 * 13 Orichalcum+
 * 1 Orichalcum
 * 1 Mythril Crystal
 * 1 Dense Crystal
 * 1 Twilight Crystal
 * 7 Serenity Crystal

Background
The Ultima Weapon Keyblade is named after the sword of the same name in the Final Fantasy series. The Ultima Weapon is typically shown within the Final Fantasy series as a long sword, with a glowing blue/white blade and a golden handle. It is usually the strongest weapon for the protagonist in the games it appears in.