Hour-Glass Chess

Hour Glass Chess has the same rules as "regular" chess, but each piece's shape is defined by a tiny hour glass that must be inverted every time it is moved. Each of the hour glasses, along with the fine grained sand, represents two different head-to-head chess pieces, except for the pawns, and the king.

For all pieces this results in grave implications, because when the sand runs through completely, the piece will find itself being supplanted by that of a lesser being - ie knights and bishops will become pawns; rooks will become knights or bishops; pawns will simply become dead skull shapes and have to be taken off the board. The queen becomes a mere rook. Only the king is immune. Of course moving them again reverses the process, which begins again.

This means that both time and motion become a component in the strategy of the game, as it may in some cases be expedient to allow a rook to "decay" to that of a knight etc.

[modified and reworded slightly for more clarity]

see Knight/pawn combo illustration