20

After proving how the position could arise by the rules of Ditto Chess, the group remains skeptical.

"We need to go back to the pub!", you shout. "I need to know if that elf is still there. He might be key to this whole mystery!"

Wesley Snow raises an eyebrow. "You really think someone you imagined playing against holds the answer?"

Despite Wesley's skepticism, David hesitantly agrees. "I guess it's worth checking out. If there's even a slight chance it could reveal what happened to Maskenissen, we have to take it."

You all gather the chess gear collected from Santa's Workshop and set off for the pub. The journey is quiet, with everyone lost in their thoughts, contemplating the surreal events that have unfolded.

Along the way, the group encounters another chessboard. "This looks like a selfmate," Magnus Carolsen observes, examining the board. "White has to force Black to deliver checkmate against their own will."

White, moving first, must force an unwilling Black to deliver checkmate. Give the sequence of moves that forces Black to mate White on the fourth move (both White and Black makes 4 moves). Here's an analysis board, and here's Offerspill's Discord server, if you need help understanding the problem.