PieceEater Chess

By Peter Aronson

Introduction

          PieceEater Chess is a game that was inspired by Donald Seagrave's game Behemoth Chess (and by the discussion of Behemoth Chess on the Chess Variant Page's comment system), a game that in turn had been inspired by Seth McGinnis and Erik Wilson's Juggernaut Chess. Like in those games, an uncapturable neutral piece randomly wanders the board, capturing pieces from both players. However, the PieceEater is a more limited monster than either Behemoth or the Juggernaut, being afraid of Kings, unwilling to retrace its path, and unable to resist eating a moving piece.

Board and Setup

          The board and setup for PieceEater Chess is identical to that for International Chess except for the addition of the PieceEater on d4 (the same location that the Behemoth starts on in Behemoth Chess):

General Rules

          The original rules of PieceEater Chess were identical to those of International Chess, except for the addition of the PieceEater piece, in-hand reinforcements, and Pawn promotion. Each of these topics are discussed in their own section below. Additionally, upon consideration and playtest, the goal of PieceEater Chess has been changed from the checkmate of the opposing King to capture of the opposing King, allowing last minute rescues by the PieceEater.

The PieceEater

          The PieceEater moves after each player's move, the order of play within a turn being: White, PieceEater, Black, PieceEater. The PieceEater moves one square each move, the direction being usually being determined by the roll of an eight-sided die, as follows:

                            +---+---+---+
                            | 1 | 2 | 3 |
                            +---+---+---+
                            | 4 |PCE| 5 |
                            +---+---+---+
                            | 6 | 7 | 8 |
                            +---+---+---+

          The PieceEater does not see the edges of the board, treating the board as a torus where the top of the board connects to the bottom of the board, and the right side of the board connects to the left side of the board. Thus, square a1 is adjacent to squares a2, b2, b1, b8, a8, h8, h1 and h2 for the PieceEater. The other pieces in the game treat the board edges as barriers as is normal.

          The PieceEater's movement is not entirely random -- there are a number of restrictions and rules:

• The PieceEater will not move to the square it occupied on the move immeadiately previous. Once it has made another move, or held still for a move, it may then move back to that square. So a PieceEater that moves from a1 to b2 after white's move, could not move back to a1 following black's move.
• The PieceEater will not capture a King, nor will it move to a square adjacent to a King (with adjacency being determined viewing the board as a torus, so a King on a1 prevents the PieceEater from moving to a1 and to any square that the PieceEater could move to from a1).
• If a piece other than a King moves to a square adjacent to the PieceEater that is neither adjacent to a King nor is the square the PieceEater just moved from, on the PieceEater's immeadiately following move the PieceEater will move to that piece's square (no die roll required), capturing that piece.

Reinforcements

          As mentioned above, each player starts with three Pawns, one Knight, one Bishop and one Rook in hand as reinforcements. If a player still has pieces in hand, they may, instead of moving a piece on the board, place one of their reinforcement pieces on the board. The Knight, Bishop or Rook in hand may be placed on any empty square on the player's first rank. A Pawn in hand may be placed on any empty square on the player's second rank. After being placed, a Pawn may on its first following move make a double-move like a Pawn that had started on the board.

Pawn Promotion

          In addition to promoting to a Queen, Rook, Bishop or Knight upon reaching the eighth rank of the board, a Pawn may optionally promote to a Rook, Bishop or Knight upon reaching the seventh rank.

Blocking Check with the PieceEater

          If playing by the old rules where victory is by checkmate, if the only piece that lies between a King and a potentially capturing piece is the PieceEater, the King is not considered to be in check. However, if the PieceEater then moves so that it is no longer blocking the potentially capturing piece, and it is then the potentially capturing piece's player's move, it is an instant checkmate. For example:

     +---+---+---+---+
     |   | k |   |   | 5
     +---+---+---+---+
     |   |   |   |   | 4
     +---+---+---+---+
     |   |   |PE |   | 3
     +---+---+---+---+
     |   |   |   |   | 2 
     +---+---+---+---+
     |   |   |   | R | 1
     +---+---+---+---+
       a   b   c   d

White moves:

R  d1-b1+
PE c3-b3

     +---+---+---+---+
     |   | k |   |   | 5
     +---+---+---+---+
     |   |   |   |   | 4
     +---+---+---+---+
     |   |PE |   |   | 3
     +---+---+---+---+
     |   |   |   |   | 2
     +---+---+---+---+
     |   | R |   |   | 1
     +---+---+---+---+
       a   b   c   d

Notes

          This game is my attempt to build a milder, tamer Chess Variant of this sort. Between the King pushing the PieceEater away, and other pieces being able to sacrifice themselves to move the PieceEater in a particular direction by moving next to it, it is partially possible to control the PieceEater's movement. The King rule also prevents the PieceEater from determining the winner purely by itself. The reinforcements and easier Pawn promotion are there to help insure that player's have enough material to try for mate, despite the PieceEater's ravages. I had considered instead increasing the powers of the pieces on the board, such as adding Wazir moves to the Knight and the Bishop, and Ferz moves to the Rook, but I decided having fewer, but more powerful pieces made the game depend more on luck, since the PieceEater could remove a larger percentage of your strength in a single move.

          Thanks to David Howe for bringing up the issue discussed in Blocking Check with the PieceEater above, and making the ASCII diagram I used to discuss it.

          After designing, publishing and playing this game with David Howe, I put it aside for a while. Then one day, I decided to try to fix the Zillions implementation's rather problematic checkmate handling. While I was in the midst of this, I realized that checkmate wasn't always exactly checkmate in this game -- sometimes you could be rescued by the PieceEater! Given that insight, I realized that this variant made more sense to play to the capture of the opposing King than to checkmate.

Zillions of Games

          I have written an implementation of PieceEater Chess for Zillions of games. You can download it here:

• piece-eater.zip

The ZRF included variants with a hostile, computer-controlled PieceEater and where each player moves the PieceEater at the start of their turn.