The Sacred Sanctum is an alternate dimension in the Pokémon World, similar to the Distortion World. It is the opposite of the Condemned Chamber. Like the latter, it can only be accessed from the Hora region.
Legend has it that all Pokémon come here when they die on Earth, as well as all good humans. It can only be accessed by using the Sacred Charts.
If Luumin is brought here, it can return to its true form of Lagralia.