Once, when I was reading the rules for Köksal Karakus' Giant Chess, it occurred to me that a royal piece that occupied multiple squares, following the rule of that game that all four squares of the giant piece must be attacked before the giant piece may be taken, would be a fun idea. After a bit of endgame analysis, however, I realized just how much firepower would be needed to checkmate such a monster and stopped working on the game. However, the idea stuck on my head, and in the course of time it occurred to me that a royal giant was the perfect excuse to put several obscenely powerful pieces on the same board. (Then again, Cavalry Chess and Tripunch Chess both suggest that no such excuse is really necessary...)
Giant-King Chess is played on a 10x10 board. The initial setup is shown below:
Each Giant-King occupies four squares. White's Giant-King occupies e1, f1, e2, and f2, while Black's Giant-King occupies e10, f10, e9, and f9. The remainder of the pieces occupy only one square each as usual.
White: Gryphon a1, j1; Rook b1, i1; Wildebeest c1, h1; Cardinal d1, g1; Knight c2, h2; Bishop b2, i2; Pawn a3, b3, c3, d3, e3, f3, g3, h3, i3, j3
Black: Gryphon a10, j10; Rook b10, i10; Wildebeest c10, h10; Cardinal d10, g10; Knight c9, h9; Bishop b9, i9; Pawn a8, b8, c8, d8, e8, f8, g8, h8, i8, j8
All pieces except Pawns and the Giant-King promote to a more powerful form.
The Giant-King occupies four squares, and may move one square in any orthogonal or diagonal direction. When moving orthogonally, the Giant-King still occupies two of the four squares it occupied before moving, whereas when moving diagonally, the Giant-King occupies only one of the four squares it occupied before moving. For example, a Giant-King occupying e4, e5, f4, and f5 on an otherwise empty board could move orthogonally to occupy e5, e6, f5, and f6, or diagonally to occupy f5, f6, g5, and g6.
If one or more enemy pieces occupy the Giant-King's destination squares, all these pieces are captured. However, the Giant-King cannot capture its own pieces, so a friendly piece on even one of the destination squares blocks the Giant-King's move.
The Giant-King cannot move into check, nor can it be left in check at the end of its player's turn. However, the Giant-King is only in check if all four of its squares are attacked. For the purposes of this rule, the front of a Giant-King does not block a rider from attacking its back as well. Thus, two Rooks or two Bishops are sufficient for placing a Giant-King in check.
There is no castling, nor any need for it.
The Gryphon is taken from Grande Acedrex, played in 13th century Spain. As in that game, it moves along an unobstructed path first one square diagonally and then (optionally) any number of squares orthogonally away from its starting square. It promotes to a Reaper, gaining the ability to move as a Rook (as seen in Ralph Betza's Tripunch Chess).
The Rook moves as in Orthodox Chess. It promotes to a Citadel, gaining the ability to move as a (1,2) or a (2,2) leaper (the move of this game's Knight). This piece can move to the same squares as the Squirk in David Moeser's Double King Chess, although that piece can make a (0,2) leap even when the (0,1) square is occupied.
The Wildebeest is an old fairy chess piece, making its most famous appearance in R. Wayne Schmittberger's Wildebeest Chess. It moves as either an orthodox Knight or a (1,3) leaper. It promotes to a Behemoth, gaining the ability to move as a (2,2) leaper or a (2,3) leaper.
The Cardinal moves as either a Bishop or an orthodox Knight. It promotes to an Archangel, a Gryphon + Bishop combination piece.
The Knight is more powerful than is Orthodox Chess. In addition to its usual (1,2) leap, it may make a (2,2) leap. It promotes to a Lioness, which can leap to any square within a two-square radius. This piece appeared in Adrian King's Scirocco, and is a Chu Shogi Lion without its double move capabilities.
The Bishop moves as in Orthodox Chess. When promoted, it gains the ability to move as sort of a diagonal Gryphon (what Ralph Betza called an Aanca in his article on Bent Riders): along an unobstructed path of first one square orthogonally, and then (optionally) any number of squares diagonally away from the starting square. This piece appeared in Tripunch Chess as the Harvester, though for the purposes of this game it is called the Chimera.
The Pawn moves as in Orthodox Chess. The initial double step and en passant capture are unchanged.
Pawns do not promote. Instead, they are used to promote the other pieces. When a pawn reaches the 8th rank, its owner may immediately promote any friendly piece. When the pawn advances to the 9th rank, its owner may again promote any friendly piece. When the pawn advances to the 10th rank, its owner may make a third promotion, and then the pawn is removed from the board.
If a player earns the right to a promotion but has no unpromoted pieces on the board, that player may instead drop any previously captured friendly piece on any vacant square adjacent to their Giant-King, in its unpromoted form. In the rare case where this is impossible, either because there are no previously captured pieces or because there are no vacant squares on which to place it, the right to promotion is forfeit.
Promotion is always optional, but it must be used immediately if it is to be used at all.
Play begins with White and proceeds as in Orthodox Chess except as already noted.