What is Dark Hex?
Dark Hex, also called Kriegspiel Hex, is Hex with hidden stones. Red still links the top and bottom edges and Blue the left and right edges, but each player sees only their own stones. The opponent's stones stay hidden until you find them.
You find a stone by playing on it. When you choose a cell that looks empty to you but already holds an enemy stone, nothing is placed: the stone is revealed to you, stays visible to you for the rest of the game, and you move again. Your opponent learns that you hit one of their stones, but not which one. This is the classical version of the game; in another version, called abrupt, such a move ends your turn instead. Every hit uncovers a new cell, so the game always ends, and as in Hex it cannot end in a draw.
The game is about building a chain while guessing where the opponent's chain runs. You can play it online with a friend, against the computer, which plays only from what it has seen itself, or on one device: a curtain covers the board whenever the turn passes, so hand the device over and let the next player reveal their own view. Spectators see no stones while the game runs. When someone completes a chain, the whole board is revealed. Any board from 7×7 to 19×19 can be chosen; small boards make for quick games.
Rule changes
| Rule | Standard Hex | Dark Hex |
|---|---|---|
| What you see | Standard HexBoth players and every spectator see all the stones on the board. | Dark HexEach player sees only their own stones and the enemy stones they have found. Spectators see no stones until the game ends. |
| Where you may play | Standard HexOn any empty cell. | Dark HexOn any cell you do not know to be taken. A cell that looks empty may hide an enemy stone; your own stones and the enemy stones you have found are off limits. |
| Playing on a hidden stone | Standard HexNot possible: you can see which cells are taken. | Dark HexNo stone is placed, the enemy stone is revealed to you and you move again. Your opponent learns that you hit a stone, but not where. |
| How to win | Standard HexBe the first to link your own two edges with an unbroken chain of your stones. Draws are impossible. | Dark HexThe same. Every hit uncovers a new cell, so the game always ends, and it always has a winner. |
| Swap rule | Standard HexOn by default: after Red's first stone, Blue may take it over instead of placing a stone. | Dark HexNot used. Blue cannot see Red's first stone, so there is nothing to take over. |
| Rex (reverse goal) | Standard HexAvailable in the table setup: linking your own edges loses. | Dark HexNot available: in Dark Hex you always play to link your own edges. |
| End of the game | Standard HexThe game ends as soon as the mover completes a chain, and that player wins. | Dark HexThe same, and then the whole board is revealed to both players and to spectators. After a resignation or a timeout the board stays hidden at the table, but the game's replay shows every stone. |
History
Games in which each player sees only their own pieces go back to Kriegspiel, a chess variant usually credited to Henry Michael Temple in 1899, in which a referee who sees the whole board tells each player whether a move is legal. Dark Hex applies the same idea to Hex, with a simpler mechanism: the board itself acts as the referee, because playing on a hidden stone reveals it. Where and when Hex was first played this way is not well documented.
The game became a subject of research in the 2010s. In 2018 François Bonnet, then at the Tokyo Institute of Technology, published a study of Kriegspiel Hex, also known as DarkHex, in the ICGA Journal. It showed that on some small boards neither player can force a win; optimal players must then mix their moves at random, and solving such a board means computing its equilibrium. In 2022 Mustafa Bedir Tapkan's master's thesis at the University of Alberta, titled "Dark Hex: A Large Scale Imperfect Information Game", improved the best known strategies for the 4×3 board.
OpenSpiel, an open-source collection of games for research, includes Dark Hex in both the classical and the abrupt version, and small Dark Hex boards are now a common test for programs that learn to play games with hidden information.