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. Skilled sorcerers are able to fold matter and distort space. Those who use the power of the Dark Dimension have a stronger control over the Mirror Dimension 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 such a method appears to be unable to trap those with the Reality Stone due to it's control over reality seemingly negating the gateway's ability to warp into the Mirror Dimension and can be destroyed with enough power, as Thanos was able to use the Reality Stone to prevent the gateway from trapping him and then crush it with the Power Stone.
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 manged 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.
Doctor Strange opened a gateway to the Mirror Dimension to protect himself against an energy blast from the Power Stone unleashed by Thanos and then send the gateway at Thanos, but it is negated and deflected by Thanos, who used the Reality Stone to prevent it from trapping him and then the Power Stone to empower the Infinity Gauntlet to be able to shatter the dimensional gate and then the Space Stone to compress the remains 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.