She finds the mirror in a closed-off corridor of the Roman catacombs. She's super not allowed to be there, but she's been reading through her mother's books and learning about her father in whispers and margin notes, and she knows he's from a different dimension, and she's read that this mirror thingy is supposed to be good for visiting other dimensions. It's super dangerous, which is why it's down here and guarded by traps, but since she can phase through stuff the getting here was kind of a cakewalk. She's not entirely sure how many dimensions there are or how one goes about narrowing down a scavenger hunt that size, but maybe the mirror will take her someplace useful, and then she'll be one step closer to figuring all of this out.
She places a hand on the mirror. It hums and lets her slide through its silvery surface, then spits her out on the other side.