The Mirror Dimension is a parallel dimension that allows the user to practice their magical abilities and fight their enemies without the public's knowledge. Those who use the power of the Dark Dimension have a stronger control over space and matter than they usually would, to the point of being able to fold and warp space in reality.
The Mirror Dimension allows an advanced sorcerer to train more advanced and dangerous spells without adversely affecting the real world, keep an eye on threats without being noticed, and a prison for those without a Sling Ring. The final usage is paramount that a prisoner is locked without a Sling Ring as it is the only means to escape. However, the realm is shown to be accessible without the use of the Sling Ring, which allowed the likes of the Ancient One and Doctor Strange to transport themselves and others into the dimension, though a Sling Ring was ultimately required to leave it. In addition, the Mirror Dimension can be used in combat by a sorcerer with the knowledge and skill needed to perform it liberally by manifesting a gateway to it to absorb attacks and then sending the manifestation at the enemy to trap them at the Mirror Dimension, but these Mirror Dimension gateways can seemingly be destroyed when enough power is directed at them, as Thanos was able to use the Power Stone to destroy a gateway that was sent towards him by Dr. Strange.
After stealing a page from the Book of Cagliostro, Kaecilius and his followers Zealots escaped through a portal to London with the Ancient One in pursuit. The Ancient One proceeded to send them to the Mirror Dimension to remove the danger they presented to the real world. In the resulting battle, several Zealots were killed by the warped landscape but Kaecilius managed to open a portal and escape with the page. The Ancient One exited the Mirror Dimension and reentered London.
Training Doctor Strange
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After the initial attack on the New York Sanctum, Doctor Strange trapped all of them inside the Mirror Dimension just before Kaecilius was about to destroy the place. Karl Mordo informed Strange that the Zealots' connection to the Dark Dimension made them even stronger in the Mirror Dimension. Realizing that their plan backfired, Mordo and Strange tried to escape. However, Kaecilius made their escape harder for them by warping reality to the point where navigation became very difficult.
While on Titan, Doctor Strange had opened a gateway to the Mirror Dimension to protect himself against an energy blast from the Power Stone unleashed by Thanos and then sent the gateway at the warlord to try and trap him only for the Titan to shatter the dimensional gate via the Power Stone before using the Space Stone to collapse the remnants into a destructive black hole that he threw at the sorcerer.
- The Mirror Dimension was influenced by the works of mathematical painter M.C. Escher and the fractal concept.