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Hydra-Vari Chess

4 times the board space and pieces, complete with a 4-headed king "hydra"! Chop off a head from the opposing king hydra, and yours will sprout a new head! Storm your king hydra and its queen out the castle gates; and checkmate the final head of the opposing king hydra, fully invade the opposing home rank, or occupy the central hill with your own hydra to win!

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Changes from standard chess:

• The game is played on a 16x16 board, equivalent to 4 of the 8x8 boards standard chess is played on; pushed together with their corners meeting in the middle. 32 pawns are set up on each player's 3rd and 4th ranks, 4 kings in the 2x2 area at the center of the 1st & 2nd ranks, 4 queens each adjacent to a king, 8 rooks total within the 2x2 areas at both far sides, 8 knights in the 2nd rank, and 8 bishops in the 1st rank. This is equivalent to 4 sets of standard chess pieces.
  
  
• The "loop rule" is in effect, that is; a pawn or piece captured goes to the capturing player's hand, where it switches to that player's color. On a later turn, that player may place it onto any empty square; instead of moving a pawn or piece they already have on the board.
  
• For playing in real life, applying the loop rule may require 8 piece sets instead of 'just' 4; an alteration is provided at the bottom for playing without the loop rule, if 'only' 4 sets are available for playing with.
  
  
• A player may leave their kings in check, unless there's only 1 last remaining king on their side; in which case, that last remaining king is subject to standard check and checkmate. Capture of an opposing king left in check is compulsory, and may not be refused or ignored. When capturing a king, you change it to your color and put it back onto any vacant square immediately; 'as part of' the capturing move, before your opponent's next turn.
  
• The fact that your last remaining king cannot step into or be left in check, takes precedence over the fact that capture of an opposing king causes you to immediately place another king of your color onto an empty square; in any situation where this may be relevant, including but not limited to when an opposing king has delivered check to your last remaining king with at least 1 guard. This may, for example, lead to the deliverance of checkmate via a king move; provided the opponent has more than 1 king remaining themselves.
  
  
• Castling becomes a jumping, capturing move; requiring only the landing squares to be vacant of allied pieces regardless of intervening pieces or check, possibly capturing up to 2 file-adjacent enemy pieces on the rank of castling. Castling may land on any pair of adjacent squares between the pieces, including either's starting square. Castling may be done in either "passing" or "meeting" manner; either "swapping" or "keeping" relative sides of the pieces involved.
  
  
• Castling not only "may" be done by queens as well as kings, but all 8 "must" castle within the 1st 64 moves (a minimum average frequency of 1 castling move per 8 moves). Any uncastled king or queen may castle with any uncastled rook sharing the same starting rank. Kings, queens, and rooks which have not castled are entirely invulnerable to capture by enemy pieces until after they do; however, may not make any move besides castling while still able to do so.
  
  
• A pawn at or behind the 7th rank may make a non-capturing jump forward along its file over any number of intervening pieces to any rank ahead of its current rank, up to the 9th rank. Only on the immediate reply turn, any capturing move to a square the pawn had just jumped over made by any opposing piece (not just opposing pawns) will take the pawn "en passant"; potentially simultaneously capturing a friendly piece the pawn had jumped over. Even after having made one or more such jumps, as long as it is currently at or behind the 7th rank, the same pawn may make another such jump.
  
  
• Standard promotion mechanics do NOT apply. Instead, on its players turn a pawn currently standing on the 16th rank may be used for any *one* of the following moves:
  
• making a potentially-capturing step to either side,
  
• where it stands; becoming a friendly version of a piece the opponent holds in-hand to make the opponent hold a pawn instead of that piece,
  
• making a potentially-capturing backwards leap along its file over any number of intervening pieces; which uniquely may capture a friendly piece to return said piece to its own players hand,
  
• or returning itself to hand.
  
• When a pawn is no longer standing on the 16th rank, it may no longer make any of these moves; until the next time it is standing on the 16th rank.
  
  
• No more than 4 pawns of the same color may occupy the same file; if a file contains 4 pawns of the same color, no pawn of that color may be placed into or capture into that file until it contains 3 or less pawns of that color.
  
• A move completing an interlocking weave of opposite-color pawns spanning the 8th and 9th ranks exiles all of the 32 pawns involved from the game on completion of the move; never to enter either player's hand again that game.
  
  
• A move causing a repetition of a position that occurred earlier in the same game; with the same moves available, same pieces in each player's hands, and same player to move; is illegal and may not be played. If all otherwise-legal moves are illegal in this manner, the game is a draw.
  
• The game is won by the player who; checkmates or stalemates their opponent's last remaining king, populates their entire 16th rank with a total of 16 of their own pawns and or pieces, or occupies the 4 squares in the exact center of the board with 4 of their own kings.

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Alterations for no loop rule:

• Capturing a pawn exiles it, while capturing a king or piece puts it in your hand without changing its color; and you may not place a king or piece from your hand onto the board on your turn.
  
• A pawn reaching the 16th rank is immediately exiled, and a king or piece of its player's choosing from their opponents hand is immediately placed on the square the pawn would otherwise have been standing on at the end of the move. A pawn may not attempt to move to the 16th rank while the opponent's hand is empty.
  
• The restriction on number of same-color pawns to a file is lifted, any number of same-color pawns may occupy and capture into a file.
  
  
• A single king stepping onto any of the 4 central squares; without stepping into check to do so, regardless of how many kings its player has remaining; is sufficient for its player to declare victory.
  
• Populating the 16th rank with your own pawns and or pieces is no longer a win condition.
  
• If by capturing a king, piece, or pawn; a player reduces their opponent to only a lone king and possibly either 2 knights *or* a(nti-paired) bishop(s); that player wins the game with the move that accomplishes this. That is to say, by capturing the last of (a piece comprising part of an entry on) this list belonging to your opponent; a 2nd king, a queen, a rook, a bishop pair, a knight triad, a knight+bishop duo, or a pawn; you win the game.