Mercenary Chess

Instructions

The main rules are in black. Additional explanation and clarification details are given in blue.

Setup:

• Mercenary Chess is played on a rectangular board. To the king's side of where black and white are setup, there are 50% more squares than a standard chess board, with all extra squares being on the same side of the board. This is gray's area of the board. This makes the board 12x8 squares.
• There are black, white, and gray sets of pieces but there are only two players, the Black player and the White player. Each of the two players take turns moving the gray pieces in addition to their own color.
• Gray is placed on the side towards the kings. This side was mainly chosen so that it wouldn't effect algebraic chess notation as much, since it just extends the current letters. However gray could be placed on the opposite side instead, if beginning too close to the kings shortens the matches too much; although it should be expected that this game will generally be more aggressive and shorter than standard chess.
• Gray has no king, but has a second queen in the king's place instead (since gray's king wouldn't be of much help anyway, and this makes the board more symmetrical).
• Some type of control markers are also required (control is discussed below). There needs to be some type of visual indicator to show which player currently controls which gray piece. Maybe you could have a black/white cap that could be placed on top of the controlled gray piece when you move that piece. The caps could be made from the tops of kings, since gray doesn't use kings they would be left over after combining two chess sets to play, and placing the cut off head of a king on top of them could represent... something; much like gray itself represents a mercenary army. Improvised control markers could be as simple as using a penny and a dime, or scraps of paper with writing on them.
• The control markers should start on top of the gray pawns that are the second closest to each file edge, with each player controlling the gray pawn that is closer to them. This means that White starts with control of the gray pawn that starts at k2, and Black starts with control of the gray pawn that starts at k7.

Rules:

• White moves one of his own pieces first, then Black moves a gray piece, then Black moves one of his own pieces, then White moves a gray piece; this pattern then repeats. So other than White's first move, a player always moves gray and then their own main color. Black gets to move gray first to help mitigate White's first move advantage.
• Gray does not have a royal piece; gray cannot be put in check or checkmate and cannot lose on its own. If gray no longer has any pieces on the board, the game simply continues as (more or less) a standard game of chess with an extended board. If there is only one gray piece left on the board and it is controlled by your opponent, you forfeit your turn to move as gray and the game moves on to the next turn. You do not lose the game, but you no longer get to move as gray. However, depending on what pieces each player still has from their main color, a resignation or agreement to draw may be recommended.
• Whichever gray piece a player last moved or is currently moving, that piece is "controlled" by that player. Each player can only control up to one gray piece at a time, and control transfers to a non-controlled piece as soon as a player touches a piece with the intention and ability to move that piece (similar to the touch-move rule). You cannot move a piece that your opponent currently controls. This keeps players from simply undoing their opponent's previous move, and it also allows one player to control a specific piece indefinitely (if they just keep moving the same gray piece every turn, their opponent will never be able to move that piece).
• Gray can be used to put another color in check and checkmate, either by itself or used as part of a combined force with pieces from a player's main color. If black or white is put into check when gray has the next move (no matter who is in control of gray that turn), the player who is in check is not required to get out of check until their main color's turn (but gray can be used to help or hurt the situation in the meantime). However if a player is put into checkmate (rather than just check), the game ends immediately. The game ends even if, for example, next turn (or the same turn) the losing player would have been able to move a gray piece that would have got them completely out of check.
• A gray piece that you currently control cannot be used directly to put your own king in check or checkmate. Controlled gray pieces cannot threaten or capture your own king, but can still threaten or capture your opponent's king. Uncontrolled gray pieces can put either or both other colors in check or checkmate. It is possible to put your own main color into check or checkmate using gray pieces that you do not control (and in the latter case, you would cause yourself to immediately lose the game).
• In summary, control of a given gray piece does three things compared to all other gray pieces on the board, it makes it so that you can move that piece, your opponent cannot move that piece, and so that that piece cannot directly put your own king in check or checkmate.
• Gray pieces cannot be used to take other gray pieces (regardless of who has control of them). All colors are their own faction and cannot harm themselves; but all factions can take pieces from all other factions (including a player using gray to take pieces from their own main color, or using their main color to take the gray piece that they currently control). If the only legal move a player can make using gray would require them to take a piece from their own main color, then they must take that piece (and vice versa, with your main color being forced to take a gray piece, even if you control that gray piece). Likewise, a player may be forced to put their own king in check or checkmate if the only legal move they can make with gray would do so.
• Black and white pawns are promoted any time that they reach the opposite side of the board's files from where they started, even if the square they reach was initially where gray or no pieces were setup. Gray pawns move along ranks instead of files. Gray pawns capture diagonally relative to their movement direction (rotated 90 degrees compared to the direction that black and white pawns capture in). Gray pawns are promoted any time that they reach the opposite side of the board's ranks from where gray started. The current controller of the pawn gets to choose what they are promoted to, and continues to control the newly promoted piece.
• All other rules are the same as the standard rules of chess, but without castling and en passant. This is to simplify the game rules a bit (since adding a third faction makes it complex enough already) and because moving gray can (sort of) be used as a replacement for those moves (plus gray might not be able to do either of those moves anyway, since it has no king and its pawns move sideways). Also castling is generally a defensive move, but this will likely end up being a more aggressive game than standard chess.

Other Possibilities:

Before the start of a new game, both players can agree to using modifications of the rules listed above. Some example modifications could include:

• allowing castling and en passant
• a player losing the game if they are unable to move a gray piece on their turn to do so (note that this could just turn into a game of "who can purposefully get all of gray's pieces captured first")
• players starting with control of a different piece, instead of the second closest gray pawn
• gray starting on the queen's side instead of the king's side (a quick way to test out this modified rule would be to just switch both the white and black king's and queen's starting positions)
• each player being able to have control of up to two (or possibly even three) gray pieces at a time; you could either control the last two gray pieces that you've moved (in which case you should start with control of just one gray piece), or, when moving an uncontrolled gray piece when you already have control of two other gray pieces, you could be allowed to choose which old piece you lose control of, and you could start the game with control of both of the two closest gray pawns (note that controlling four or more at a time could result in a fundamental change to the way players need to think about the game, by making players need to worry more about how to utilize and preserve their extra pieces, rather than worrying about how to manage the control itself and how to deal with uncontrolled threats, but controlling two or possibly three could add more strategic depth and coordination between your main force and gray)

Mercenary Chess could also be played with additional unusual rules such as being able to move around the starting positions of your pieces (like in Pre-chess or Fischer Random Chess), while taking turns placing gray pieces (using a pattern that's similar to the starting turn order). You could use some form of time control, but the exact time that's allotted may need adjusted compared to standard chess; the time for a player to move both gray and their main color could be combined into just one time-frame. You could also add a forth faction (of a different color) that would function similar to gray but on the opposite side from where gray starts, or have two armies that are both gray and players could choose which one they would like to move a piece from (although I'd guess that limiting the game to only having a third army/faction will likely work better).

Mercenary Checkers likely wouldn't work as well as Mercenary Chess. In chess, even if a gray piece is just sitting there unmoved, it can still help to put your opponent in check or checkmate. In checkers this isn't the case; only pieces that you're actively moving or are right next to one of your opponents pieces really matter.

Click and drag the pieces to play.