Fluidity Chess

Inspiration has come in 2021. I've created this game for Lichess (free libre chess site with millions of players where I started playing chess more seriously), but its developers don't embed new variants (currently), so I've written it here. It hadn't Pawns and rangers' restrictions and was very fast game. 
Then I realized (with editors' help) that game is quite solved, and remastered it in summer of 2023.

The main source of inspiration is Atomic Chess, which is playable on that site. I play it well btw.

Setup

Standard chessboard and setup.

Pieces

It's illegal to move to an occupied square. However, pieces deliver check (attack foe King) as they do in chess, but only on their move destinations. They capture differently.

Pawn

• moves & checks as in chess:
  • moves one space straight forward, can move two from start rank;
  • checks opposite King if he is diagonally in front of it;
• is very cheap stepper and captures
  • a horizontally adjacent piece by bypassing — stepping diagonally forward to the space directly beyond this foe piece & deleting it from the board (same as en passant in Chess, but unlike other pieces here);
  • an adjacent piece which's directly in front (only if your pawn is on start rank) by going through it to next space forward;
• as very cheap stepper, it can take only one piece per turn;
• can't move through friendly pieces;
• and promotes as in Chess, on the farthest rank to any piece but the King.

Bishop, Rook and Queen

• move & check as in chess: unobstructed diagonal, orthogonal and both lines, respectively;
• are rangers and capture opposite pieces by cutting through them, even if they are adjacent to each other, to any free square behind the farthest of them;
• as rangers, they can capture up to 3 pieces per turn (this is not the greatest number of pieces can be taken, but a manual restriction for making game longer);
• and can jump, but only to take, so they can't slide through friendly pieces.

Knight

• moves & checks as in chess, L-shape;
• is leaper and captures foe pieces which are in 1 and 2 squares orthogonally in direction of this move (as we learn his moves, 2 squares straight in one side, 1 in other; and first 2 squares are affected by capture there);
• as leaper, can capture up to 2 pieces per turn, and checks so differently than captures;
• is only piece whose capturing of foe pieces is optional (so it can take just one of two targets or not take any of them).
• and can jump over any pieces (without affecting friendly ones).

King

As in chess, 1 square in arbitrary directions, can't move into check; but he doesn’t capture by himself because he can't cut through (and by this, two kings can be near each other, due to being not in check. But you can checkmate or stalemate opponent’s king near your king by another piece(s)).

Castling: you can castle if:

• your King and Rook haven’t moved yet;
• your King isn’t under check;
• your castling path is free from your pieces & isn't under check;
• you have opponent’s pieces between King and Rook, but not on adjacent spaces. (So if you have foe pieces on b & d files, you can castle queenside yet).

and if you castle through them, you capture them — up to 1 piece by castling kingside, and up to 2 if queenside, and castling can capture only minor pieces, i. e. Bishops or/and Knights.
King can capture only during castling. By the way it's the only case when moving to occupied squares is legal, i. e. they firstly become cleared by King or Rook and then you step on them.

He is royal, but can't be captured by displacement, you simply should avoid check and keep him safe.

Rules

Your aim is to checkmate the opposite King. However, stalemate is a loss for the player without legal moves (and thus it's another way to win). You also can capture him by cutting through if: 

• opponent will block the path of check after your move, and his King will be behind his own pieces (and behind the king will be free square(s)); 
• you can do it by your knight or pawn.

This move is always legal, even if you're in check (or unavoidable check). Same logic in Atomic chess - they can take your King but no one can give an order to do so. (This is why King can be called General).

Draw occurs if:

• only 2 kings (& 1 bishop) are left on the board (unless it's win in 1 when losing King is in corner, advanced one stands on adjacent diagonally square, and you move by Bishop to space near them both, and that's stalemate);
• there is 3-fold repetition;
• or there are 25 moves without checkmate/stalemate/capture.

2 kings and 1 knight endgame is not draw.

Examples

Let us assume that all pawns are traded.

In this diagram, Rook on d1, Bishops on a4 & b2, and both Kings cannot be captured at this turn because there's no squares behind them (at least in directions which are possible on this turn). Circles show legal moves for white Rook on b4. It can move vertically (but not to b1 due to friendly piece on b2 blocking its path), but this isn't pleasant here. However, it can capture black's Knight on c4 and Queen on d4 at once (landing on e4), or take them with Bishop on f4 also (three pieces are permitted to be taken at once) landing on g4 and checking black King; so it's win in 2 for White (because King can't take and will have to go to h5/h3, and Rook from d1 can come [back;)] to h1 and checkmate).

Also white can move the Bishop on b2 (which's pinned by the way and whose legal moves are shown on diagram above) to c3 (but it's not pleasant here) or to take the Queen, only (landing on e5 or f6) or with Knight on g7 landing on h8 (note that if there was black piece on e5 or f6 it could be taken also, but if there were two black pieces on these squares, that capture wouldn't be available at all: there would be more than 3 pieces); so this taking makes the situation unclear: initiative goes to black (Knight can take the b4 Rook by going to a3 with hidden threat to white King which should move to b2 to save the game), but this white Bishop is lucky and breaks their plans to win, with possibility for d1 Rook to 'dance' with black King for draw.
Also white can move the Rook on d1 (also possibly capturing Queen by landing behind her) or the King.

But if white does so or it's black's turn, this is won game for black. If white moves the King, it'll be punished by the Queen moving to now free a1 (and taking Bishop on b2) with immediate checkmate. 
This diagram shows legal moves for black Knight on c4. Red circles in green ones are marking the squares to which the Knight can move either with or without capture. It can simply move to b6, d6, e5, e3 and d2. Field b2 isn't available because it's occupied, even by non-friendly Bishop. Also this Knight can move a3 or a5 with or without capturing white Rook on b4 (but friendly Bishop on a4 is safe). This position is win in 2 if the Knight moves to a3 (taking the Rook or not doing so) because it can't be taken by white Bishop, which by the way occupies the only square where the King could find his shelter. This threat is hidden but fatal for white (anyway its King will get captured on the next turn even if white moves and checks the black King. White King cannot move into covered b1 and isn't pleased to move into threatened a2).

So initiative still decides who wins;)

Notes

Knight is the only piece who can capture a piece on every square. Pawn cannot take only pieces on 1st/8th ranks, rangers cannot take cornered pieces, Bishop even doesn't take pieces on edges. No word about King;)

Knight now worths 4 pawns.

This is playable on Game Courier (uncoded).

Last revised by Lev Grigoriev.