Okay, Mirror Open
Well... alright, I guess that's all it takes.
No, none of the keys do anything, you do a sweep of all the places you've been before, including the chapel and whatnot, but nothing turns up. Then you remember...
You never explored level 6 of the castle. The Museum Level. Just never went in.
In you go. The place is a disaster. Signs of combat... blast marks, mainly, and some dead monstrous skeletons. The level's been ransacked, with a few Library-esque magic glass cabinets now open and empty.
A thorough sweep does turn up a scrap of paper in a dustbin. Picture of a helmet, and written beneath it in fancy script is: "ANTITHESIS".
...
You type the word in, and the helmet icon flashes, disappears, leaving behind a second glowinng mirror icon. To activate the portal and head on through, just press the button...