Unlike Diancie, Hoopa, and Volcanion, Magiana is not part of the Pokémon X & Y and Pokémon Omega Ruby & Alpha Sapphire games' original game data. To put it simply, it still unknown on how to obtain this Pokémon in-game. Some fans speculated that this is the first confirmed Pokémon for the 7th generation of games.
[Source: Bulbapedia, Serebii]