Dark Aura

Dark Aura is one of Riku's abilities. They come in two different ways. The first Dark Aura is teleporting around the battle field with his weapon in front of him and ending by landing on the ground with a hard slam that creates a shockwave. This ability is first seen in Kingdom Hearts (game). His second Dark Aura is shooting a barrage of dark energy orbs at his opponents. The second version of Dark Aura is not to be confused with the previous versions with the same name from Kingdom Hearts or Kingdom Hearts: Chain of Memories.

Kingdom Hearts: Chain of Memories
''Description: Rush enemies repeatedly with blade in hand. Finish off by stabbing Soul Eater into the ground for a column of light that confuses the enemy. Only available in Dark Mode.''

It reappears in Chain of Memories, but this time as a Sleight. It also is a sleight in Kingdom Hearts Re:Chain of Memories. In Chain of Memories, you learn it after Riku fight Ansem for the first time, it is only avaliable in Dark Mode.

This is the Attack Card requeriment for Dark Aura

Kingdom Hearts II
''Description: Stops enemy movements with Dark Magic spells as commanded when Sora's combo is about to end. MP Cost:10''

It is avaliable after Riku become a partner in The World That Never Was. Riku fires multiple Dark Orbs that resemble Dark Firaga at the enemy. It can also be used by the player when Riku becomes playable in the final battle against Xemnas.

Kingdom Hearts 358/2 Days
In Kingdom Hearts 358/2 Days it also appear in the Final battle but this time Riku's weapon is covered with light and he don't need to use it with Dark Mode.

Trivia

 * The Dark Aura in Kingdom Hearts II is very similar to Dark Firaga from the previous games.