Dark Mode

Dark Mode is a special ability Riku can transform into Kingdom Hearts and 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. Riku Replica can also use this form too.

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.

Kingdom Hearts
After coming into the service of Maleficent, Riku slowly began to lose his heart to the darkness. The witch granted Riku several new abilities, such as power over the Heartless and even Dark Mode, itself. Riku utilized this dark power to attack Sora, the Keyblade master having only the Beast on his side after Donald and Goofy had left him to follow the Keyblade on King Mickey's instructions. Donald and Goofy later returned to Sora's side when they realized how strong their bond was, however, a battle with Riku ensuing. While Sora and friends regrouped, Riku, still in Dark Mode, fled via a dark portal.

Riku eventually reverted to his normal form, severely weakened. He met a hooded Ansem, the being persuading Riku to open his heart to the dark. It was then the enigma possessed Riku, forcing him to permanently retain Dark Mode. Riku, now under the influence of Ansem, had gained power over the Keyblade of People's Hearts. The possessed boy unlocked Maleficent's true darkness, the witch falling at Sora's hand as he had planned. Now nothing stood in the way of Ansem claiming Kingdom Hearts as his own. He used Riku to attack Sora in an attempt to reclaim the last of the Princesses of Hearts' hearts, Kairi's. Riku lost his body and his soul during this event, Using his remaining strength to fend off Ansem while Kairi, Donald, and Goofy fled Hollow Bastion after Sora sacrificed himself.

At the End of the World, Ansem still took on the form of Riku's Dark Mode. He later did away with this form, however, before battling Sora in his true appearance. Dark Mode was then lost to Riku from this point forward.

Kingdom Hearts: Chain of Memories
After his reencounter with Ansem in Castle Oblivion, the dark denizen tampered with Riku's darkness, activating Dark Mode once again. Riku would utilize this form as he struggled in the card worlds. Riku would change into Dark Mode sparingly, a massive fear of the darkness manifesting in the depths of his heart, much to Ansem's displeasure.

Soon Ansem wasn't the only one who sought to claim Riku. Vexen, Lexaeus, and Zexion, members of the mysterious Organization XIII, wished to use the boy and his dark powers to halt the plans of the traitorous Marluxia. When Vexen challenged Riku, it was his immense power that led to the creation of the Riku Replica, an aggressive and protective replica of Riku created by Vexen from the boy's combat data. Ansem seized the Replica as an opportunity to use it as a model for Riku, urging him not to fear the dark like the fake.

Eventually Riku came before Lexaeus. A battle ensued between the two, Ansem possessing Riku's body in order to do away with the Nobody. Riku's sudden recollection of Ansem's control almost allowed the dark man complete control over his body. Were it not for the King, Ansem's plan to get Riku back under his control would have succeeded.

After the failures of both Vexen and Lexaeus, Zexion was forced to take action. He tormented Riku with illusions in the card world of Destiny Islands, taking on the guise of Sora and swallowing Riku in the light. It was here that Naminé appeared in the form of Kairi, teaching Riku not to fear the darkness and not to run from the light. Riku then regained himself, controlling and transforming into Dark Mode willingly in order to destroy Zexion. Riku no longer feared the darkness from this point forward.

In the card world of Twilight Town, Riku was approached by DiZ, the bandage-faced man telling him to seek out Naminé. He battled his replica a final time, meeting up with the girl at last. She offered to put him to sleep so she could lock his heart, freeing him from Ansem and the consequences Ansem had on Riku's heart. Riku refused, needing his power to finish Ansem for himself. It was then Riku headed towards his final battle.

Riku ventured into Castle Oblivion. Battling Ansem for the last time, the dark influences held upon him faded, Riku losing Dark Mode with no chance of its return, the closest to being free of Ansem he had ever been.

Appearance
In Chain of Memories, 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 Ansem 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.

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. It should be noted that Dark Mode is only triggered when the player manages to fill up the Dark Points gauge up to 30. Once Riku is in Dark Mode, he can use it for an indefinite period of time, but he loses a certain amount of Dark Points with every broken card. So if Riku takes out a 2 card and the enemy takes out a 9, Riku loses 7 DP. When his Dark Points go back to zero, Riku reverts back to his normal form.

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

 * Vanitas from Kingdom Hearts Birth by Sleep 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.
 * Dark Mode makes a cameo appearance in the opening AMV of Kingdom Hearts II, Riku seen fighting Ansem while Sora duels Marluxia back-to-back. It also appears during the AMV at Hollow Bastion, battling Sora before offering a hand to the Keyblade master while a large wave crashes down. Dark Mode's last appearance was Roxas's dreams in the beginning of the game.