The mirror realm exists beyond your mirrors and anything that contains glass. 
Luminato - House of Mirrors-0801

if two mirrors are in the same trajectory multiple reflections will be seen however to enter into this realm the one who looks into the mirror has to concentrate on the other reflections of his, at some point the reflection will have a mind of itself and it will pull the victim into the mirror to the mirror realm.

the mirror realm is filled with glass stairs and more mirrors, if one of these mirrors are entered the experiencer will find himself at somoen elses house.

If he or she were to stay in the realm, humanoid glass creatures will attempt to escape from where the experiencer entered in a violent matter the experiencer may end up getting stabed during this processe by glass pieces and then they would escape and attack the nearest humans that see them and multiply by transforming theyre victims into other glass humanoids, they were taken down with bullets and sharp objects. 

if you ever find yourself in this realm then exit to the nearest mirror as fast as possible and break the mirror. 

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