An Unversed Keyblade Wielder whom feeds off the negative emotions of all living beings, unlike others who control the Heartless and Nobodies, he and his kind can create the Lesser Unversed to do their bidding and if defeated, the Lesser Unversed's Sentiments and Power return to its creator.
In an alternate universe to the original Kingdom Hearts, it has been two thousand years since the three Chasers wared against two of their own, Xehanort and Darkfire. Darkfire absorbed the Darkness of his twin brother: Ventus, therefore becoming the Pureblood Unversed, an Elite humanoid Heartless named Vanitas. Like Sephiroth is to Cloud, Vanitas is forever a part of the Darkness in Ventus and his descendents therefore his Darkness is presently connected to Prince Roxas' Darkness, descendent of Vanitas and Ventus.
Eventually when Roxas was pulled into the "End of the World", Roxas's Darkness became too strong and extracted itself from the young Keyblade Wielder allowing Vanitas to once again be reborn while sensing the wayward Unversed of Terra and Aqua, Sora had gone in to save Roxas and so his Darkness too gained a mind of its own before making the Unversed Arashi (Japanese for Storm while Sora means Sky) which Vanitas too sensed nearby.
Arashi caught Vanitas whom thought he was falling into the Dark Realm after being defeated as Shadow Roxas however Arashi managed to reach him and turn him upright, Arashi corrected Vanitas by saying that they were not rising falling into the Realm of Darkness but in fact rising to the Realm of Light and Arashi explained that he was the Sentient Darkness of all of Terra's descendents, therefore gaining a lot of control over the Darkness as well as knowledge of each generation.
The two of them finally took form at the Keyblade Graveyard where the first Keyblade War had taken place and where Xehanort and Vanitas had been defeated along with their army over two thousand years ago, Arashi immediatly used his knowledge of a place to save them time, taking Vanitas and himself through a Dark Corridor to Castle Oblivion which he claimed as his own.
There Arashi made an interesting offer that put Vanitas on a path away from his Master and probably sealed the fate of Xehanort once and for all, why should Vanitas listen to Master Xehanort's goals of controling this world when Arashi and himself could gain the power of Light and Darkness as puppets for their rule on all creation, Vanitas took quite an interest to this plan before Master Xehanort arrived in a young body with Contremisco and Fluctus.
Vanitas learns from Master Xehanort that he had gone out and fought a small battle against the Armies of Light at the Mound of Judgement where he managed to forge new Keyblades from the Forge located in that Mountain.