to both solve the mystery of some grisly murders (including her sister's) and the mystery of herself/her past, Emilia was on the hunt for something called the Triple Moon Mirror, a mirror belonging to the goddesses that she has a divine connection to. When properly activated, the mirror can show you the past, the present, and the future upon request. Its magic is dormant atm, of course, as I promised Ruxi it would only ever be used sparingly/with permission.

but I put it in her inventory even before all this in-game mirror madness began, so I feel like it's just Fate