Tacu's Enigma: The hardest chess problem

Advent of Chess 2025 just finished. We saw 24 chess compositions like helpmates, selfmates, proof games, retrograde problems, and other unusual chess problems. 2419 participants solved a combined 24,090 problems in what became the biggest Advent of Chess yet. If you missed out, you can still log in and solve all 24 of them late thanks to our new interactive board for submitting answers.

While most of these problems were challenging for those new to unorthodox chess problems, one really stood apart as the undisputed Final Boss. Offerspill’s Discord was full of confused puzzle-solvers, people spending hours and even days trying to figure it out. We even extended the deadline for this puzzle; while other problems needed to be solved within 24 hours to grant a point, this one was granted a full 4 days, ending at midnight on December 28th.

Before we take a look at Tacu's Enigma, the Final Boss, we'll take a look at a related, more accessible problem. To solve a Tacu's Enigma, we first need to understand Proof Games.

Day #16: Proof game in 7.5 moves

In a "Proof Game", the goal is to reach a position within a certain amount of moves. On Day 16, we were presented with this problem by Andrew Buchanan: Starting from the initial position, give a sequence of moves that reaches this position after White's 8th move (8 moves for White and 7 moves for Black). If you want to have a go at it without any hints, you can log in at Advent of Chess and input your solution on the interactive board.

Proof games can look overwhelming at first, but they can be solved through a process of elimination, and often by looking at White’s and Black’s paths separately. Our most telling clue is the missing bishop on c1. With the pawns on b2 and d2 still there, we know that the bishop was captured on c1. But why which piece?

Let's first consider the knight on b8. It would take it 4 moves to get to c1, and then 4 more moves to get back to b8. That's already more than our 7 allotted moves, so that can’t be it!

The only other piece that can reach c1 is Black's missing queen. It would take the queen five moves to reach c1, one move to play d6, and then one more move before it got captured. This looks promising! Now that you've deduced Black's path, I'll leave it up to you to figure out White’s side. You can try solving it on the interactive board here.

Tacu's Enigma

Now that you know how to solve proof games, let's take a look at Tacu's Enigma, invented by Romanian chess composer Vasile I. Tacu, and first published in the magazine Europe Échecs in 1991. It's a much bigger deductive challenge where you have to piece together which pieces are where, along with how they got there in the designated number of moves.

This is also a Proof Game, but instead of knowing the final position, you only know where there are pieces. All pieces, both White and Black, are displayed here as black dots. Starting from the initial position, find a sequence of moves that reaches this position after Black's 6th move (6 moves for White, 6 moves for Black), where White can then deliver checkmate on the next move (move 7). Your strongest clue here is the checkmate in 1. Ask yourself: Where could the pieces be to allow for White checkmating Black on the next move? You have to simultaneously consider possible squares for the Black king, the configuration of the surrounding pieces, and where the checkmating White piece(s) must be to be able to deliver the mate.

Only 15 Advent of Chess participants managed to solve this in the first hour, and 356 after 4 days. For comparison, the Proof Game on day 16 had 134 solvers in the first hour, and 787 by the end of that day. And keep in mind that many of the first solvers are literally world-class solvers. gmsolver (GM Kacper Piorun), on top of the leaderboard, is six-time winner of the World Chess Solving Championship.

What's next?

Advent of Chess is over, but you can still solve all the problems late, or give up to see the solutions if you're stuck.

• Attempt Tacu's Enigma (Day 24)
• Advent of Chess 2025 Wrapped (Check out the community stats!)

Stay tuned for an upcoming series where we deep-dive into more of these unusual chess problem categories.