13

Bells still ringing in your ears, you descend the staircase of the bell tower. Reaching the bottom, you see the church doors wide open, flapping in the wind, yet no one is in sight.

Outside, you see fresh footsteps in the snow. Assuming they belong to the mysterious figure from the bell tower, you start following them. You suddenly come across someone lying face down in the snow. You help them up and realize it's the Austrian winter sports equipment manufacturing company founder (and part-time house-elf) Dobby Fischer! As he regains his senses, he explains that he was tackled by someone.

Dobby points to a nearby gate, locked with a chess-themed mechanism. "Whoever tackled me was in a hurry to lock this," he says. "The only way to open it is by solving a chess puzzle." He leads you to a chessboard conveniently set up beside the gate. "Unfortunately, I'm the only elf on the North Pole that has never touched a chessboard, so you're on your own," Dobby says.

This is a reflexmate problem. This means that White, moving first, must force an unwilling Black to deliver mate – with the added condition that either player must checkmate whenever a mate in one is possible! In other words: It's like selfmates, with the addition that if either player can give a mate in one, they must.

Under these conditions, with White to play first, give the series of moves that forces Black to checkmate White no later than on Black's 4th move (both White and Black alternates, making 4 moves each). Here's an analysis board. For more information, read the Wikipedia article on reflexmates. If you're still confused, ask in Offerspill's Discord.