The mirror dimension is, as its name states, a dimension WITHIN the Night Dimension. You can enter it by entering a mirror in a dream/nightmare. It creates a parallel universe, where everything is upside down, literally and also figuratively. If you have good qualities, the bad ones will show prevalent in their mirror version. The mirror version of someone, is simply called REFlECTORA.
The mirror dimension is considered your UNCONSCIOUS PSYCHE. It's why your mirror cannot escape the dimension, unless you summon or somehow get it out of there.