Dark Mode

Dark Mode is a special ability Riku can to transform in Kingdom Hearts and Kingdom Hearts: Chain of Memories. Riku initially receives this ability from Maleficent, and uses it against Sora after Sora reclaims the Keyblade, but loses it after his body is taken by Ansem. After entering Castle Oblivion, Ansem manipulates the darkness in Riku's heart, giving Riku the ability to use Dark Mode freely. In this form, Riku can use his darkness without the worry of being taken over by Xehanort's Heartless. Because Riku is so darkness-oriented, this form gives him a substantial advantage in battle. While unconfirmed, it is very likely that when Riku takes the form of Xehanort's Heartless, he is using this form, able to tap into his darkness to the point of turning into Darkness itself.

Appearance
When Riku activates this form, a blue aura surrounds him, he jumps into the air and shouts, "Darkness!". A bright blue light shoots out of his chest, and in a flash of light, his clothes transform into the same suit he wore while possessed by Xehanort's Heartless in Kingdom Hearts. This consists of a form-fitting suit that appears to be made of black muscle tissue. The black fades to red around the wrists and comes with purple gloves that are tucked under the suit. The suit has a purple-blue color on the chest and part of the torso with the Heartless emblem on the chest. It has purple feet and shins that fade into black legs. It has a dark purple belt and a cloth around his waist, opening in the front. The cloth is an off-white color that fades into dark purple and is ripped all around at the bottom.

During his second fight with Roxas, Riku morphs into the form of Xehanort's Heartless. It is thought to be a stronger version of Dark Mode, being able to use the Darkness of Xehanort's Heartless, but at the cost of his bodily appearance. After the explosion of Ansem the Wise's Kingdom Hearts Encoder completely destroyed the last of Xehanort's Heartless, the Darkness lost its form, freeing Riku from Xehanort's Heartless forever and physically freeing him from the Darkness as can be seen when he tries to summon a dark portal but fails to.

Abilities
In Kingdom Hearts: Chain of Memories and Kingdom Hearts Re:Chain of Memories, since Riku is a playable character, he has a specific moveset while in Dark Mode. While as Xehanort's Heartless, he presumably has all of his powers.

Attack Sleights

King Mickey helps Riku while he is in Castle Oblivion, allowing him to use these moves:

Friend Sleights Duel Sleights

Trivia

 * Many fans see Riku's Dark Mode as an equivalent to Sora's Drive Forms.
 * It is unknown if Riku is able to use this form now that Xehanort's Heartless is gone, it is highly unlikely.
 * Master Xehanort's Apprentice wears a suit similar to that of Riku's in his Dark Mode, except that the colors are slightly different - they are mostly hues of black and red.