Hidden boss will get shadow crystal after each chapter (it will not be displayed after jevil, but there will be dialogue), and then there will be a special dialogue with the old businessmen in the main city, suggesting that the next hidden boss needs a prop called "shadow cloak".
The triangle rune shadow crystal, through which you can see the body of dark things, will become a glass in the bright world, and through this glass you can see the darkness of bright things.