The Mirror Realm, also known as the Mirror World, is a dimension that exists between Mirror Bridges.
The Mirror Realm is a dark reflection of the Material World. In it, everything is the opposite of the world in the state you enter in.
Trapped in the Mirror World
During her plan to steal the Alexandria Cell, a large battery rumored to be used by the Order of the Library of the Neitherlands, Harriet Schiff discovered that the Librarians were instead using the ground up remains of Fairies to power their spells.
When Harriet attempted to escape with evidence of the Order's transgressions, Gavin teleported beside her, throwing her into the Mirror Bridge she used to reach the Library and smashing it, destroying the bridge and trapping her in the Mirror Realm.