Birth By Sleep FM

He's the 2nd bonus boss you can fight after clearing the last episode. Go check out the Land of Departure on your best file, and prepare to be owned.

To add on, you have to have cleared all 3 stories, plus clear last episode. Once you've completed last episode, you then have to beat LSV in the new landing area at keyblade graveyard. Beating LSV can be done on any file, and once its done, that same file can then go back to land of departure to get owned by mysterious figure.

inb4 Organization XV

Nah, this guy predates the organization.

 Spoiler

minor spoiler, but Nomura stated that the fight with MS is canon to the story, so unless we're suddenly time travelers, the organization hasn't even been thought of at this point.