Kingdom Key D

Mickey's Keyblade is a Keyblade used by King Mickey. It is an alternate version of the Kingdom Key, called the Keyblade of the Realm of Darkness. This Keyblade of the Realm of Darkness was found there.

It did not always belong to the King. Before Kingdom Hearts, he previously wielded the Keyblade Star Seeker, as shown in Birth By Sleep, most likely when he was still an apprentice of Master Yen Sid. In addition, this Keyblade was shown in the secret ending of Kingdom Hearts II called The Gathering, along with Sora's Kingdom Key and Riku's Way to the Dawn. Notably, these three Keyblades, along with Master Xehanort's keyblade, are the only ones in the movie shown with Keychains. It is unknown exactly how the King obtained the Keyblade of the Realm of Darkness by the end of Kingdom Hearts.

He does not have any sort of weapon, except for his wits, in Timeless River.

It was originally seen in the closing video for Kingdom Hearts. It is expected to appear in Kingdom Hearts coded.

Magic
Strangely enough, Mickey manages to unleash the magic of light while using the Keyblade of Darkness, speculating that a light shines even within the deepest darkness. He can use it to unleash Pearl to ward off his enemies and use Healing Light to fill up his Drive Gauge. Should he succeed to fill up the gauge, he can fully restore Sora's HP and MP. However, should he fail half-way, he can only revive half of Sora's HP and the MP gauge remains untouched.

Trivia

 * Since this keyblade is the inverse of the regular Kingdom Key it can be assumed that when a keychain is attached to alter it's form the keyblade, such as Oblivion, would have inverse colors.
 * This Keyblade's title remains unknown so far, which makes fans to speculate titles of their own: some call it the Inverse Kingdom, or Kingblade. Some even call it Darkside as it comes from the Realm of Darkness.