Ashur - Idea 2

It's a mirror that contains a complete replica of our reality, interacting with this reality in any way can cause detrimental effects to our own, and is only able to be interacted with from this side. The mirror usually takes the form of an ordinary wall or door in the other reality, unable to be seen by any of the mirror-world's inhabitants. once something passes through the mirror, it is unable to return on its own.