Shadow Chess

shadow_chess.md

Shadow Chess

Version 1 (2017/11/15) by @TetroniMike and @paniq

Beware: This is an alpha version. The rules are completely untested.

Shadow Chess (also known as Quantum Chess, Imaginary Chess or M.A.D. Chess) works like regular chess, but with additional rules that are designed to produce new strategical opportunities and interesting outcomes:

1. You'll need two boards and twice the set of pieces. The squares must be big enough to hold several pieces.

2. In addition to the regular board, there's now a second board called the shadow board. The shadow board is placed alongside the main board, with a second set of chess pieces, called the shadow pieces, set up in the same starting configuration.

3. When a player makes a move on the main board, he must also move a shadow piece of the same class and color on the shadow board. The shadow piece can be moved independently from the main piece, but the same movement constraints as for the main board apply.

   Example 1: White moves a pawn two steps forward. White then moves an arbitrary shadow pawn one step forward.

   Example 2: Black moves a rook three steps horizontally. Black then moves one of the two shadow rooks to the end of the shadow board vertically.

4. Shadow pieces can't take other shadow pieces. (but they can indirectly take pieces on the main board with a collapse, which is explained further down)

5. Multiple shadow pieces of either color can occupy the same square.

6. When a main piece takes another main piece, the taken piece is observed and must reveal a shadow piece of same class and color. The player whose piece was taken selects the shadow piece to be revealed. If no matching shadow piece is left, nothing happens.

7. When a shadow piece is revealed, it teleports from the shadow board to the main board, but remains at the same location and thus turns into a main piece.

   Example: A shadow bishop at E5 is revealed. The bishop piece moves from E5 on the shadow board to E5 on the main board.

8. When, after a reveal, two pieces or more end up on a single square, two situations may occur, separately or together: a collapse and/or an entanglement.

9. A collapse happens when two or more main pieces of separate color end up on a single square. In this case, the color of the square selects which piece gets to live, and which gets taken. If white and black pieces collapse on a black square, the white pieces are taken and the black pieces remain. Likewise, if they collapse on a white square, the black pieces are taken and the white pieces remain. The reveal rule is applied to each taken piece.

10. An entanglement (or merge) occurs when two or more main pieces of the same color end up on a single square. In this case, the pieces become forever entangled and can only be moved together; effectively they now form a single piece, a chimera.

11. A chimera exists only on the main board, and can be moved by the rules of any of the pieces it consists of.

    Example: A chimera consisting of a rook and a knight moves three steps horizontally, like a rook. The next turn, it moves two squares vertically and one square horizontally, like a knight.

12. When a chimera is taken, all of its pieces are taken, and the reveal rule is applied to each taken piece individually.

13. A castling move can only be initiated on the main board, and is not followed by a shadow move.

14. A promotion can only be initiated on the main board. It also removes a shadow pawn of the acting player's choosing and adds a new shadow piece of the same class and at the same coordinate as the new main piece.

15. The game ends when both kings of the same color have been taken.

you could manage the hidden board like the hidden movement in fury of dracula. card pairs and an extra set of pieces for board coordinates. When you move a piece, you reveal its old location and take those coordinates back into your available deck.