Mate in One the Missing King The post &quot;A mysterious mate in one&quot; made me think of one of my last problems which is also a Mate-in-One with no black King:
    [Title &quot;White to move and mate in one&quot;]
    [FEN &quot;1B1NB2K/P2RN3/8/1R6/1P5P/4P2Q/1PP2PP1/8 w KQkq - 0 1&quot;]

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.
Your post is perfectly fine here, my friend. Chess problems and puzzles are on-topic here, and we do need more. All kinds of all sorts are welcome. We just need to bring them here!
I think that out of the 14 open squares on the board, only:

 placing the Black king on both a6 works!

This is because placement on the squares:

 a1, a2, a8, a4, b1, c1, c4, d1, d2, e1, e2,  f1, f6, f8, g1, and h6 all result in an illegal position. This is due to the fact that Black either has no last legal move if it is White's turn, or it would've been impossible for the Black king to get around the current White pawn structure. Thus, they are not options at all. This explains the &quot;retrograde-analysis&quot; tag.

Therefore, remaining the square works:

 with the solution checkmate being 1. Rd6#.