A mysterious mate in one

Here's a question that's a little off the usual format, but problem questions like this have been done before and are on topic, so here we go!

Question: Can you find the mysterious mate in one? The trick is not board rotation, removing a piece, retracting any moves, completing a move, or an illegal move-there is but one solution. Have fun solving!

`````` [Title "White checkmates Black in one move"]
[FEN "5nRQ/6PR/1p6/p2P4/p1B5/3Pp3/8/1b1K4 w - - 0 1"]
``````
• Stupid question probably - wheres the black king? Nov 6 '20 at 13:53
• Looks like the point of the problem is not to determine where the black king is, but to find the solution without explicitly knowing where the black king is Nov 6 '20 at 21:28
• How about mate in zero? "Ref, the opponent has absconded with his King piece, I demand he be disqualified IMMEDIATELY" Nov 7 '20 at 20:50
• Why not just state explicitly that the black king is invisible? It's an interesting puzzle, but imo the ambiguous wording is unnecessary and annoying.
– MaxD
Nov 8 '20 at 19:41
• @RewanDemontay Honestly no idea what you're asking.
– MaxD
Nov 8 '20 at 20:17

Looking at the problem as if the black King is currently invisible...

gxf8=Q# All squares are now covered. So where ever the black King is hiding, the game is over.

• rot13(Jryy, nyy fdhnerf rkprcg gur juvgr xvat'f fdhner q1 :c)
– Stef
Nov 7 '20 at 17:01
• @Stef, gur fdhnerf jvgu oynpx cvrprf naq cnjaf ner nyfb abg pbirerq, evtug? Fb vs gur oynpx xvat jnf abg vaivfvoyr ohg znfdhrenqvat, guvf jbhyqa'g jbex... Nov 7 '20 at 18:27
• Is there some kind of cipher I am missing here or are you two simply trying to “out-gibberish” each other? Nov 7 '20 at 23:41
• @fartgeek ROT13 is a simple substitution cipher. Replace each of the letters A-M with the letter 13 places ahead of it in the alphabet, and each of N-Z with the the letter 13 places prior. So Stef's "Jryy" becomes "Well". It was used on Usenet to give the answers to puzzles, but in such a way that you wouldn't accidentally see the answer and spoil it for yourself - you had to use your reader's "ROT13" button to read it. Nov 8 '20 at 0:36
• @Nfgevq_Erqsrea Nu V frr... Nov 8 '20 at 1:10

A solution may be that it used to be possible to

promote to an enemy piece, as mentioned e.g. here. This loophole has long been fixed in the official rules, but under old rules it was allowed to do `1. g7xf8=(black)K` which would be mate.

Would that really explain

the tag?

Perhaps, in the sense that

we do need to go back in time.

• (+1) Seems this question would be more appropriate in Puzzling, especially with the meaning of retrograde here. Nov 6 '20 at 9:16
• It seems that if this were the answer, it would be a very ugly problem with so many irrelevant pieces. Maybe we are supposed to place a black king somewhere? Nov 6 '20 at 9:24
• @RewanDemontay: I also wondered about the black king. Maybe the problem is just a chess zen koan: how to mate in one, when there's no king? Nov 6 '20 at 13:51

This is easy.

The Black king is somewhere on the 8th rank. It is checkmated in one move by pawn x knight on f8, queening. Now the new queen is giving check, and the rook on h7 prevents the king from going anywhere, resulting in checkmate.

• This post (sort of) gives the correct answer but an incorrect explanation (I’m being deliberately vague here. I have suggested an edit that hides the answer behind a “Reveal spoiler” button.). Please read the accepted answer. Nov 7 '20 at 12:51
• Understood. My apologies. Nov 7 '20 at 20:36