Dark Mode is a special form Riku can transform into 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 Ansem. Because Riku is so darkness-oriented, this form gives him a substantial advantage in battle, such as increased speed and feeding the Soul Eater's unique power-up ability with the power of Darkness. Riku Replica can embrace the darkness willingly, thus he can also use and stay in Dark Mode.
In A Super Mario World
Dark Riku, simply referred to as Riku, appears as the true antagonist in A Super Mario World. In the second episode, after Mario and Link escape the castle with Princess Zelda, Cloud Strife prepares to fight the greater ffoe that Bowser had warned them about before being killed. Riku is then suddenly revealed to be the mastermind behind everything. Cloud is at first disappointed (as he thought the foe was Sephiroth), but they still engage in a fight to the death that continues in the final episode. He ultimately defeated when he and Cloud deliver their final blows at the same time, but only Riku is affected by it, and he crumbles into bloody pieces.