Three Realms Chess

This is a 'small boards' variant of Dragonchess where all pieces (though not all pawns) can reach any of the three boards. Like Dragonchess, most of the pieces have different abilities depending on which board they are on, with pieces generally being at their most powerful on their home boards.

I was initially trying to make an 8x8x8 variant, but triagonal riders only being able to reach 1/4 of the squares while also being infuriatingly good at slipping past pawn walls in the opening was something I found to be un-fun. By making the board not a cube, I could get away with letting pieces use triagonals without these problems.

Setup

Three Realms Chess is played on 3 standard chessboards: the Earth, the heavens and the underworld. The heavens use the standard file labels (a-h), the Earth uses the 8 letters in the middle of the alphabet (j-q) to label its files, while the underworld uses the last 8 letters of the alphabet (s-z) to label its files.The letters i and r are not used as file names.

Here is the setup for the heavens. Note that the colours are inverted.

This is the setup for the Earth. It may look familiar to some players.

This is the setup for the underworld.

Pieces

Definition of terms

Forwards: Towards your opponent, and away from you.
Up: From the underworld to the Earth to the heavens
Down: from the heavens to the Earth to the underworld

Orthogonal: One of the 6 cardinal directions: forwards, backwards, left, right, up, down.
Diagonal: Any combination of 2 non-opposite cardinal directions. There are 12 diagonal directions in total.
Triagonal: Any combination of 3 non-opposite cardinal directions. There are 8 triagonal directions in total.
Orthogonally/diagonally/triagonally x: One of the cardinal directions you use must be x. 
(a,b) leaper: Choose 2 non-opposite cardinal directions that are not up or down. Move a spaces in one of the directions and b spaces in the other, ignoring any pieces in the way.
(a,b) rider: Choose 2 non-opposite cardinal directions that are not up or down. Move a spaces in one of the directions and b spaces in the other, ignoring any pieces in the way. You may repeat this move as many times as you like until encountering another piece of either colour
(a,b,c) leaper: Choose 3 non-opposite cardinal directions. Move a spaces in one of the directions, b spaces in another and c spaces in the third, ignoring any pieces in the way.

Move passively: Move the piece to an unoccupied square in the appropriate direction.
Capture: Move the piece to a square occupied by a piece of the opposite colour in the appropriate, then remove the opposite coloured piece from the board.
Move: Grants the ability to move passively or capture on the specified squares.

The Earthly Pieces

Pawn: Passively moves one space orthogonally forwards, captures diagonally forwards. A pawn on n4 can move to n5, or capture on m5, o5, e5 or w5. Upon reaching the opponent's back rank, pawns promote to Grandmasters. The pawns' movement does not change between realms. Pawns may also make an initial double step and get captured en passant as in FIDE chess.

Rook: Moves any distance orthogonally, including up or down. The rook's movement does not change between realms.

Knight: On Earth, a knight may move: as a (2,1) leaper; one space orthogonally, diagonally or triagonally downwards, controlling the 9 spaces below it; or one space orthogonally upwards. In the underworld, a Knight may only move one space orthogonally upwards, returning to the surface. In the heavens, a Knight may move passively as a (2,1) rider, or leave the heavens by moving as a (2,1,1) leaper.

Bishop: On Earth and in the heavens, the bishop may move any distance diagonally. However, in the underworld, a bishop can only move one space triagonally. A bishop that enters the underworld must change the colour of square it's on on the way out.

Queen: On Earth, moves any distance orthogonally, diagonally or triagonally. In the heavens and the underworld, moves one space orthogonally, diagonally or triagonally.

King: On Earth, moves one space orthogonally, diagonally or triagonally. In the heavens and the underworld, moves one space orthogonally. While not quite as much of a sitting duck as he is in Dragonchess, the King is in serious trouble if he is forced out of the Earth. The King may also castle with a Rook as in FIDE chess.

Grandmaster: Moves any distance orthogonally, diagonally or triagonally regardless of which realm it's in. There are no grandmasters in the initial starting position; the only way to get one is by promoting a pawn.

The heavenly pieces

Honorable Horse: moves 1 space orthogonally forward then 1 space diagonally forward. It can leap over a piece on the square orthogonally in front of it, but cannot stop on that square. An honorable horse on n4 can reach m6, o6, e6 and w6. Upon reaching the back rank, honorable horses promote to a pegasi. An honorable horse can never reach the seventh rank, as its movement does not change between realms.

Silver General: Moves one space diagonally, or one space orthogonally or triagonally forwards. Its movement does not change between realms.

Pegasus: Moves as a (2,1,0) leaper in all realms.

Angel: In the heavens, an Angel may move any distance diagonally, or one space orthogonally or triagonally. On Earth, the angel may only move any distance diagonally, like the Bishop. If an Angel enters any space in the underworld, it transforms into a fallen angel.

The underworld pieces

Imp: When in the underworld, moves one space orthogonally or triagonally forward. When on Earth, moves one space orthogonally down. An Imp can never reach the heavens, so it has no movement powers there. Upon reaching the opponent's back rank, imps promote to fiends.

Warmachine: When in the underworld, moves any distance orthogonally like a Rook. When on Earth or in the heavens, it moves up to 2 spaces orthogonally, leaping over any piece in its way.

Scholar: Moves any distance triagonally, leaping over any piece in its way, or makes a (2,2,1) leap. The scholar must change which realm it is in every time it moves, though its movement does not change between realms.

Elephant: When in the underworld, moves any distance diagonally like a Bishop on Earth. When on Earth or in the heavens, it moves up to 2 spaces diagonally, leaping over any piece in its way. The elephant is the only colourbound piece.

 Fallen Angel: Has the same movement capabilities as the last piece the opponent moved. If the opponent's last move was a promotion, the angel imitates the newly promoted piece. A fallen angel may sometimes try to imitate a piece with no movement options (a pawn, imp or honorable horse on the 8th rank, an honorable horse on the 7th rank, in imp in the heavens, or an angel that has just fallen). If this happens, the fallen angel cannot move. Fallen angels cannot be involved in castling, even if they imitate a King or Rook. Fallen angels cannot make an intial pawn double step, even if they are on the 2nd rank. Fallen angels do promote if they reach the back rank while imitating a pawn, imp or honorable horse.

Fiend: Moves up to 2 spaces orthogonally, diagonally or triagonally, leaping over any pieces in its way. Its movement does not change between realms.

Rules

The rules of FIDE chess are used with the following exceptions:

The boards representing heaven and the underworld should be placed on either side of the chess board representing Earth (where the chessboard representing Earth is placed according to Article 2.1) and rotated such that each player's near left corner is a light square. The heavens should be to the left of the player playing with the white pieces.

If a player touches one of their Rooks with the intention of castling and castling is a legal move, they may castle by capturing their own king (ignoring Article 3.1) then moving both pieces to their appropriate squares. This rule change isn't necessary for this variant, I just don't like how FIDE disallows castling as a rook move.

Note that rule 3.9.2 (don't put yourself in check) forbids moving any piece that will give the fallen angel the ability to attack the king. As a fallen angel's movement capabilites are determined on its opponent's turn, a fallen angel cannot deliver check.

A draw may be declared under Article 5.2.2 (draw by insufficient material), Article 6.9 (draw by timeout vs insufficient material) or Article 7.5.5 (draw by too many illegal moves vs insufficient materal) if the relevant king (both kings for Article 5.2.2; the king of the player whose flag fell for Article 6.9; the king of the player who made a second illegal move for Article 7.5.5) is on Earth and can neither get checkmated while staying on Earth nor get forced to leave Earth. Moves where the King leaves Earth voluntarily shall not be considered.

When using algebraic notation, the following letters may be used: no letter=pawn (W may be used instead if a letter is necessary) A=Angel, B=Bishop, C=Scholar, D=Warmachine, E=Elephant, F=Fiend, G=Grandmaster, H=Honorable horse, I=Imp (M may be used instead if capital I may be confused with a lowercase l indicating an l-pawn), K=King, N=Knight, P=Pegasus, Q=Queen, R=Rook, S=Silver general, X=Fallen angel

Notes

A bare king on Earth can deliver checkmate against a king not on Earth by moving triagonally adjacent to the enemy king then bullying it into the corner.

The pieces in the central 4 files in the heavens develop very quickly, so control of the heavens is important in the early game. On the other hand, the underowld pieces are slower to develop but have powerful 2-space leaps, making them stronger in the endgame.

Relative piece values

Pawn: 1
Imp: 1
Honorable Horse: 1⅓
Silver General: 2
Fallen Angel: 2
Warmachine: 3
Elephant: 3½
Bishop: 4
Knight: 5
Scholar: 5
Angel: 6*
Pegasus: 6
Rook: 6*
Queen: 8*
Fiend: 12*
Grandmaster: 14*

*this piece is sufficient mating material against a bare king on Earth. All other pieces are not sufficient mating material, unless they can successfully promote. I do not know which combinations of minor pieces can collectively form sufficient mating material, as it is not intuitive. For example, 2 pegasi are sufficient mating material, even though 2 2D knights are not, because the 2 knights forced stalemate forces the king off of Earth when done by pegasi.

Last revised by Andrew L Smith.