If you’re playing on PC, each piano key is marked with a number you tap on your keyboard to play that note. The order for those is 5, 4, 2, 3, 4. Solving the puzzle opens a secret door.

If you don't know how to play the piano, you can use your blacklight to reveal the same Cyrillic notations above the piano keys. If you play the correct sequence of notes, a secret door will open ...