You cough as small clouds of dust rise up off the floor with every little movement. This particular room probably hadn't been used since your great great great grandfather's time or something because your footsteps leave deep imprints in the thick layer of dust. Almost like snow, if you want to compare the gross scene in front of you to something a little nicer to look at.
Having finished your usual chores and duties, you thought it would be fun to explore around the castle but so far your mini adventure was a lot more boring than you expected it to be. No secret passages or interesting trinkets lying around forgotten. Just damp hallways and dusty forgotten rooms. Normally a servant was not allowed to just walk about as they pleased, but you were actually from a family of lesser nobles that served the royal family. After generations of loyal service, even the King and Queen held a certain fondness for your bloodline and were valued as much as family. There were, of course, certain lines that a servant was still not allowed to cross but a bit of mischief here and there was often tolerated.
You're about to leave and close the door behind you when something catches your eye. There's a large sheet covering a tall and thin piece of... Furniture? A painting? There's plenty of things thrown around that's covered like that, but this one catches your eye because there isn't a speck of dust on the clean white sheet covering it. You wonder if someone moved that over there recently... But then there should've been tracks left on the floor.
Reaching out, you pull the sheet off and gasp. It's a mirror, about 8 feet tall in height. You've honestly never seen something so big made of glass before (except the church windows, but they were made of fragmented pieces of glass so it did not count) and you wonder how much it probably cost to make such a thing. You reach out to admire the silver frames, when a hand suddenly plunges out of the mirror and grabs your wrist. You were so occupied by the mirror itself that you hadn't noticed a glaring difference between you and your own reflection. Or at least, what was supposed to be your reflection.