OK so this would be more of a variant "for fun", the idea is standard chess but pieces have a non-zero probability to decay into other pieces, a bit like atomic nuclei can undergo radioactive beta decay.

The "conservation rule" being that the "value" of the sum of the pieces needs to be conserved through the process.

If a piece is completely surrounded on all sides then it cannot decay.

But if out in the open*...
- A queen could decay into a rook, bishop/knight, and a pawn
- A rook could decay into a bishop/knight and two pawns,
- Minor pieces could either not decay, or decay with low probability into three pawns

Algorithmically, the piece would disappear, and of the empty nine unoccupied contiguous tiles, the three "spawn points" would be chosen randomly.

As all "decay rules" result in three pieces, this means that a minimum of two empty adjacent tiles need to be there for the process to take place.

Decay is evaluated at the start of each turn, I'd say a parameter between 1% (1 in 100) and 2% (1 in 50) for each of the pieces of the player, where allowed.

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A more elaborate decay rule could also be considered:
- A queen could decay into a rook, bishop/knight, pawn, or three minor pieces, or two rooks and a pawn of the opposite colour
- A rook could decay into a bishop/knight and two pawns, or two minor pieces and a pawn of the opposite colour

Of course this would be a nightmare to play OTB given the need of having extra pieces, calculating probabilities at each turn, and so on, but should be a piece of cake to do in code!

Seems like reverse version of the game Battle of Kings, available on SchemingMind.

Example game: www.schemingmind.com/home/game.aspx?game_id=753812

When you click on the info tab above move list, the full rules are explained.

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Cool, thanks for mentioning that!

Yes there could be a deterministic way to implement this as well, for example queen always decays on turn 10 (for both players), rooks decay on turn 15 and turn 20, and so on. But I think the random version could be more fun.

Another type of decay could be a "lossy" decay where you have always a loss of one pawn.
The decay rules would be:
Q [worth 9] -> R B/N [worth 8] or B/N B/N P P
R [worth 5] -> B/N P [worth 4]
B or N could not decay, or decay to P P [worth 2]

This might work even better as you'd need one fewer free adjacent spaces.
(Except the Q -> B N P P one ofc)

The game name could be "Decay Chess" or "Piece Decay Chess" or "Random Piece Decay Chess" as there is nothing "radioactive" about this.

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A diagram to show how a possible Queen decay could happen from a realistic position:


("5" is at b3, where the queen is)

Edit to add: logiqboard.com/ seems to have the type of sandbox environment to try this out, and if not registered the only limitation is that a game must last less than 60 minutes!

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