The post "A mysterious mate in one" made me think of one of my last problems which is also a Mate-in-One with no black King:
[Title "White to move and mate in one"] [FEN "1B1NB2K/P2RN3/8/1R6/1P5P/4P2Q/1PP2PP1/8 w KQkq - 0 1"]
All the white pieces are there but none of the black ones.
White (at the bottom) to move.
The position is legal, except that the Black King (and the Black King only) is not displayed. Find it - there is but one place.
Mate in one.