That mirror is totally the mirror of Erised. we even had a Harry Potter reference earlier in the episode.
What was the HP reference, POM? I must have missed that.
I wonder if the mirror is like the hat. The more pieces that are added to more powerful it becomes. I wonder if the pieces of the mirror come from different magical items. Like maybe pieces of a looking glass that can help her travel between worlds or see into other worlds. She had known have learned about Emma somehow.
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Nothing is impossible on OUAT. Everything has a way of repeating itself. Keeper of the Keeper tread: https://oncepodcast.com/forums/topic/official-keeper-thread-3/#post-217924