15

After completing the problem set up by David Snowell and his group, David steps forward. "You're finally ready," he says. "Please take a seat."

"We were the ones who stopped Grinchuk last time," David begins. "He almost destroyed the spirit of Christmas back then. We have managed to keep him away for a century. And now, with Maskenissen missing, we fear he has returned."

He explains that they've been secretly observing you since the day you played chess against Wesley Snow 9 days ago. Looking around, you recognize Wesley and other familiar faces among the group in the underground cabin. "You've shown remarkable skill in solving the problems you encountered after leaving the pub. We realized you were on Grinchuk's trail, but we didn't think you could face him alone. That's why we intervened."

Before David can continue, your attention is diverted. A burst of laughter from a nearby chess game between two elves catches your ear.

You turn around, and see two of the elves being in the middle of a friendly chess game.

"Hold on," the elf playing White says. "I just made a move, but I had a chance to checkmate in one instead!" The surrounding elves, including David Snowell, gather around the board.

The elf looks at you. "Can you figure it out? How could I deliver a checkmate in one instead of the move I played? I know you started watching after I made my move, but given that I could mate in one instead of what I did, you should be able to figure out my last move as well!"

In short: Take back White's last move in this position, and give a checkmate in one instead. Two moves are needed as part of the answer: First the move that White just played instead of checkmating in one, and then the checkmate in one that could have been played instead. Note that when retracting a capture, the captured piece needs to be stated as well – an example would be to write Kb3xRc4 if we're taking back king on b3 takes Rook on c4.

Here's an board editor, and if you need help understanding the problem, ask in Offerspill's Discord server (there's a #discussion channel for Advent of Chess).