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(Fairy chess is a general term for any kind of chess variant with pieces that aren't used in the normal game)

(Inspired by The Black Prowler (chess))

In this variant, white has their entire army against one black Mastermind. The Mastermind has the following special rules:

  • It can start on any space that one of White's pieces doesn't start on.
  • It can't move on its turn.
  • On its turn, it must make a single legal move for White, with one exception: it can capture white pieces with this move.

For example, a possible opening is 1. e4 Kxd1.

In addition, one additional rule for White: White's king acts as any other piece and can be captured as any other piece.

White wins when they capture the Mastermind.

I've recently updated the win condition for black due to complaints in comments: black wins when White can't capture the Mastermind no matter what, unless Black has to force a three-move repetition in order for this to happen. (A three-move repetition is still a draw.)

Assuming best play, what is the outcome of this game?

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  • $\begingroup$ If the Mastermind moves a white pawn, will it move in the same direction as when white moves it? Also, are there any rules prohibiting repetition? $\endgroup$ – Bass Oct 8 '18 at 13:10
  • $\begingroup$ @Bass yes, and nothing prohibiting repetition (a draw is possible that way) $\endgroup$ – Excited Raichu Oct 8 '18 at 13:13
  • $\begingroup$ @ExcitedRaichu If a draw is possible by repetition then what exactly is the win condition for black? If white repeatedly checks black with one piece an black repeatedly undoes the move, what is the outcome of the game? $\endgroup$ – Bolton Bailey Oct 8 '18 at 16:01
  • $\begingroup$ Can you clarify the requirements for black to win? If all that's left is the mastermind vs the king, is that a win for black because they can just undo white's moves forever, or a draw? $\endgroup$ – user2357112 Oct 8 '18 at 17:22
  • $\begingroup$ Any hints? Are we close at all? $\endgroup$ – jafe Nov 2 '18 at 11:15
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Partial answer:

The mastermind cannot lose.

This can be achieved by the following strategy:

1: Place the Mastermind on the fifth rank, e5 for example

then

* whenever white moves a (non-pawn) piece, move it back. Because white cannot capture white pieces, this completely cancels white's move, and is always a legal move, except in the case of castling. Since white cannot get the other pieces out of the way, castling won't be possible anyway.

To cope with the pawns,

* whenever white gives check with a pawn, move that pawn
* whenever white moves a pawn to the seventh rank, move it forward and promote it to a knight.

Against this strategy,

white can only dream of winning if he could somehow discover a double check with a pawn move. This is impossible by the virtue of placing the Mastermind in a position where this cannot happen. So white should be playing for a draw.

@Hexomino raises a good point in the comments, so

against pawn moves that weren't explicitly listed, the Mastermind should pass. Since that is not allowed, he can do the next better equivalent thing, and move the white king along the first rank. This will never increase white's attack lines, rather it will remove them since another white piece is likely to get captured.

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  • 2
    $\begingroup$ What if white gets all its available pawns on the 6th rank with a move? What should black do? $\endgroup$ – hexomino Oct 8 '18 at 13:33
  • $\begingroup$ Really, I'm nitpicking as it seems like there will always be a way to play this. +1 from me anyway. $\endgroup$ – hexomino Oct 8 '18 at 13:43
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Looks like

both sides can force a draw, so with best play the game is drawn.

Strategy for black

The mastermind always has at least a draw with this strategy:
- Start on the 8th rank.
- If white moved a major piece, move it back to where it was.
- If white moved a pawn and it's not yet at the 7th rank, move the king somewhere on the 1st rank (taking pieces if possible).
- If white moved a pawn up to the 7th rank on the same file as the mastermind, move the king somewhere on the 1st rank (taking pieces if possible).
- If white moved a pawn up to the 7th rank on a different file, move the pawn forward and promote to a knight.

White ends up with some major pieces on their home squares, a pawn in front of the mastermind, and a bunch of knights on the 8th rank, none of which can capture the mastermind in one move.

Strategy for white

I think that black can't prevent either a black knight or bishop from getting to an unprotected checking square to force a repetition. So I believe that white can force a draw, although I haven't gone through all the possible variations.

Note that if black is to win, they can only make captures and pawn moves to make progress. Anything else and white can simply undo the move with surefire repetition in three moves unless black captures or makes a pawn move. (Or castles, I suppose. Would be a pretty odd move for the mastermind player to castle, but let's not rule that out.)

Let's say the mastermind is on a dark square. White can start by bringing forward the knight which is next to the dark-squared bishop, i.e. 1. Nc3. Now, black has the option of allowing the knight forward or taking it with a pawn.

Allowing the knight forward leads to a surefire repetition, as there is no way to cover all checking squares by making pawn moves and captures only. Black has no reliable defending pieces – every piece except the light-squared bishop can themself deliver check, and even if black gets the light-squared bishop into a square where it covers the knight's checking squares, all black has to do is move the bishop away and black is forced to repeat.

Example with mastermind on b8:
1. Nc3 Bxg2
2. Na4 Bc6
3. Nc5 Bb5 (the bishop now boxes out the knight)
4. Bc6
and black is forced to move the bishop back to avoid checks.

(White doesn't even have to wait that long – 2... Bc6 can be met directly with 3. Bg2.)

Taking the knight allows white to bring in the attacking bishop into a checking square. Example with mastermind again on b8:

1. Nc3 bxc3 (dxc3 Qd8+)
2. Ba3
and black cannot prevent Bd6+.

Or with mastermind on h8:

1. Nc3 bxc3
2. Ba3 g4
3. Be7 g5
4. g6
and black cannot prevent Bf6+.

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