Mirror Dimension

"You are now inside the mirror dimension, ever present but undetected. The real world isn't affected by what happens here. We use the mirror dimension to train, surveil and sometimes to contain threats. You don't want to be stunk inside here without your Sling Ring."

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. It is known for rapidly distorting space and landscape, which sorcerers that visit it can control to an extent. 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 the physical dimension.

The Theft
After learning that Kaecilius and the Zealots have stolen a page from the Book of Cagliostro, the Ancient One travels to London to confront them.

She proceeds to send them to the Mirror Dimension and kills several Zealots using the warped landscape, but fails to prevent Kaecilius from escaping with the page.

Training Doctor Strange
While training new recruit Doctor Strange, the Ancient One brings him to the Mirror Dimension and explains its unique properties.

Battle of the Mirror Dimension
Doctor Strange and Karl Mordo enter the Mirror Dimension to escape Kaecilius. However, Kaecilius makes their escape harder for them by warping reality to the point where navigation becomes hard.