23

The cavern hums with unstable energy, light and sound warping unnaturally. The group works frantically at the console, the Chrono-Sleigh vibrating with power as it draws from the black hole’s tether.

David’s scanner emits sharp beeps as he types rapidly. "Grinchuk’s interference has corrupted the TSC. If we don’t sever the sleigh’s connection to the black hole now, the timeline will fracture."

Santa steadies the group with his calm presence. "Then we finish this. No matter what it takes."

Grinchuk stands near the sleigh, his grin unwavering. "Finish it? You’re playing with forces beyond your comprehension. Everything you’ve done—everything you think you’ve saved—rests on a thread I’m about to sever."

Suddenly, the cavern fills with flickering images, fragments of time spilling into view. Scenes from past and future ripple across the walls. A bustling pub filled with elves, laughter, and the clinking of mugs appears. The words North Pole Pub glow above the door.

Magnus tilts his head. "Where is that? It says North Pole Pub, but I’ve never seen it before."

David frowns, his voice low. "It doesn’t exist… not yet. The plans for that area are still ice storage. If we’re seeing this… it’s from the future."

The image flickers again, briefly showing a shadowy figure at the edge of the pub. Grinchuk, his face twisted into a wicked grin, vanishes as the vision fades.

Santa studies the vision, his gaze heavy. "Not just the future. A future where Grinchuk returns."

David’s scanner emits sharp beeps. "Grinchuk’s interference has corrupted the TSC. One last task remains—we need to sever the sleigh’s connection, or the timeline fractures."

Santa nods. "Then let’s finish this."

The console flares brightly, projecting a final chessboard into the air. David studies it, his expression tight. "This is it—the TSC’s stabilizer logic. It needs proof that checkmate can happen on the 7th move. If we solve this, we reset the field."

This is a helpmate problem, which means that both players cooperate to achieve mate.

With Black to move first, how can White, with Black's assistance, mate on the seventh move? Black plays 7 moves and White 7, and White's 7th move is mate.

Please go to the Lichess analysis board, carry out the moves, and copy the PGN under the board. Make sure there are no alternative variants in the PGN before you copy (no parentheses). If you're confused about anything, join our Discord server.