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
In order to unlock the Ultima Weapon for synthesis, all other recipes must first be synthesized at least once.
 * 5 Lucid Gems
 * 5 Power Gems
 * 5 Thunder Gems
 * 3 Mystery Goo
 * 3 Gales

Kingdom Hearts Final Mix

 * 5 Thunder Gems
 * 5 Mystery Goo
 * 3 Mystery Force
 * 3 Gales
 * 3 Dark Matter

Kingdom Hearts II
The Ultima Recipe can be found in the Basement. 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.