Three Doors are all that stands between Apocalypse and the rest of the world. A series of obstacles must be completed before each door can be opened.
For the Third Door, it must be opened from the inside. This can be done by a shapeshifter like Mystique, because her powers allow her to transform into something small enough to fit through a tiny crack in the door. Then, they must put their hand on the Mystical Stand, and that will immediately open the door, at the cost of turning the subject to stone.