Kingdom Key D

The Kingdom Key D, also known as the "Kingdom Key -Dark Side-" (キングダムチェーン ダークサイド), is the default form for Mickey's Keyblade, introduced in Kingdom Hearts. It is a Keyblade of the Realm of Darkness and the dark counterpart of Sora's Keyblade from the Realm of Light. In Kingdom Hearts 358/2 Days, it can be upgraded to the Kingdom Key D+, the Kingdom Key D++, and the Kingdom Key DΩ.

The Kingdom Key D was originally seen with Mickey in the final scene of Kingdom Hearts, and appears throughout Kingdom Hearts Chain of Memories, Kingdom Hearts II, and Kingdom Hearts 358/2 Days as King Mickey's Keyblade. Before the events of Kingdom Hearts, when he was an apprentice of Master Yen Sid, King Mickey instead wielded the Star Seeker Keyblade.

Kingdom Hearts
Mickey enters the Realm of Darkness in search of the dark counterpart of Sora's Kingdom Key in order to help Sora seal the Door to Darkness. After teaming up with Aqua, Mickey realizes that since Sora acquired his Keyblade in the Destiny Islands, its counterpart has to be located in the remains of the Islands. Eventually, Mickey finds the Kingdom Key D behind a door in the Depths of Darkness, then uses it to slay a troop of encroaching Darksides and seal the Door to Darkness.

Kingdom Hearts 358/2 Days
Mickey briefly brandishes the Kingdom Key D when he encounters Riku in Ansem's form, but dispels it when Riku reverts to normal.

Kingdom Hearts II
Upon Sora's awakening, Mickey continues to wield the Kingdom Key D on his mission to restore the light. Upon Xemnas's defeat, Mickey does not use the Kingdom Key D again for some time, switching back to the Star Seeker.

Kingdom Hearts 3D: Dream Drop Distance
Upon learning that Xehanort's Organization XIII would soon pose a threat, Mickey resumes wielding the Kingdom Key D. He uses it to cast Stopza on the Real Organization XIII.

Kingdom Hearts III
It is revealed that because the Kingdom Key D comes from the Realm of Darkness, Mickey can use it to open doors between the light and dark realms.

When Riku and Mickey enter the Realm of Darkness to search for Aqua, they are attacked by a Demon Tower, which damages the Kingdom Key D and snaps Riku's Way to the Dawn in half. Retreating to the Mysterious Tower, they inform Yen Sid of what happened to their Keyblades, who then helps power up the Kingdom Key D and get a new Keyblade for Riku. This results in the fusion of the Kingdom Key D and Star Seeker, creating the Kingdom Key W.

Design
The Kingdom Key D is a mirror of the Kingdom Key, and this purpose is reflected in its coloration - where Sora's Kingdom Key has a silver blade and Keychain and a golden guard, Mickey's has the opposite color scheme. Even the rainguard, which is blue on Sora's Kingdom Key, is red on Mickey's, although both Keyblades possess a dark-colored handle.

After being damaged in Kingdom Hearts III, Mickey has the Kingdom Key D improved. It features a gold handguard, and a blue blade patterned in the manner of a starry sky. The point where the shaft joins to the rainguard now has a diamond-patterned bell shape. At the four corners of the handguard are five-pointed star designs. The Keychain is unchanged.

Gameplay
In Kingdom Hearts II, the Kingdom Key D is used whenever Sora faints during a boss battle and King Mickey enters play. However, it lacks a finishing move, so King Mickey is unable to finish off any of these bosses; he must instead revive Sora in order for the boss to be defeated.

In Kingdom Hearts 358/2 Days Mission Mode, the Kingdom Key D is King Mickey's default Keyblade, and does not require gears to be equipped. The ground combo consists of a diagonal upward slash from the right leg, followed by the same slash from the left leg, and finishes with a 360° spinning slash. The aerial combo is the exact same as when he uses the Star Seeker Keyblade, consisting of a horizontal slash to the right finishing with a vertical slash upward, followed by the upward slash leading into a 360° spin with the Keyblade held above the King's head, and ends with a series of three thrusts as he descends.

Another side, Another story...
The Kingdom Key D appears in the secret movie Another side, Another story..., where it is wielded by Mickey during his brief appearance.

The Gathering
The Kingdom Key D also appears in the secret movie The Gathering, where it is pulled from the ground by Aqua at the Keyblade Graveyard.

Kingdom Hearts Birth by Sleep
The "Lightseeker" (ライトシーカー) was originally intended to be a Keychain for Ventus, and although it is not available in the final game, the data for the Keyblade's icon and stats remains within the game's code. If equipped through the use of a cheating device, it grants Ventus +3 Strength and +2 Magic. The Keyblade's model is present in gameplay when equipped in the original Japanese release of Kingdom Hearts Birth by Sleep, but the model is removed in later releases of the game.

Kingdom Hearts 3D: Dream Drop Distance
The Kingdom Key D is listed as one of Riku's Keyblades in the game files, though it is unusable in normal gameplay. Like every other equippable Keyblade in the game, it changes appearance when visiting The Grid, taking on a black, grey, and light blue coloration.

Face My Fear
The EP cover art for "Face My Fears" depicts Utada Hikaru wielding the Kingdom Key D in her right hand.