Astral Plane

The Astral Plane is a plane of existence that can only be accessed by those with Psychic powers.

X-Men: Apocalypse
Xavier used his previously established connection with Apocalypse to bring the all power Mutant into the Astral Plane. While Charles briefly managed to gain the upper hand Apocalypse managed to easily overcome and mercilessly beat Charles. However Jean having finally gained complete control over the power of the Phoenix managed to defeat Apocalypse in both the Astral Plane and the real world.

Legion
Oliver Bird was trapped in the Astral Plane somehow for years where he eventually met fellow Mutant David Haller.