“All things age. All things die. In the end, our sun burns out. Our universe grows cold and perishes. But the Dark Dimension, it’s a place beyond time.”
The Dark Dimension is a location in the Marvel Cinematic Universe that exists in the multiverse. Time as seen in timelines of the multiverse does not exist in this dimension.
Some magic users are able to draw power from this dimension and use it as Dark Dimension Magic. It is also home to the source of energy named Darkforce. The Dark Dimension is ruled by a powerful being called Dormammu. It is also where entities like Loa and exiled sorcerers like Morgan le Fay reside. Furthermore, the dimension has a dark world like Hell, home to the Spirit of Vengeance.
A portal to the dimension.
The Dark Dimension is held at bay by the Sanctums which are three bases of the Masters of the Mystic Arts, an order of sorcerers that use the Time Stone to protect the Earth from mystical threats, including from the invasion of entities from the Dark Dimension. The Sactums were once destroyed by the Zealots, a group of sorcerers led by Kaecilius, that tried to bring the Dark Dimension and Dormammu to Earth. They were stopped by Doctor Strange, who used the powers of the Time Stone to trap Dormammu in a loop, forcing him to accept Strange's bargain of leaving Earth forever and take the Zealots with him, banishing all of them into the Dark Dimension. Following this, the destroyed Sanctums were restored.
After the events of Avengers: Endgame, with the Time Stone being destroyed by Thanos after he accomplished his goal, the Masters of the Mystic Arts are left without their weapon to defend Earth against the Dark Dimension and the entities that reside inside it. A year after Thanos' defeat, Doctor Strange caused an incursion within the multiverse so Clea visited him and opened a portal to the Dark Dimension so they could fix his mistake.
