active king chess

• standard chess board and setup.
• a turn consists of 2 moves: a non-king move and a king move. when it's a player's turn he has to make a move with a non-king piece, and a move with the king.
• a king cannot move into the square it was in in the previous move. (it can move into a square it was in two moves ago, however.) if no other move is possible for the king, that player loses the game.
• if any player cannot do one of these two moves, he loses the game.
• there's no checking, checkmate or stalemate. if the king is captured, since the player will not be able to complete the two moves, he will lose. if a player is left with a bare king he will also lose the game as there is no non-king piece to move.
• there's no castling since the king would have moved by the time it can castle.
• otherwise standard chess rules apply.

en passant chess

• standard chess board and setup.
• a queen, bishop, rook or pawn can be captured by en passant if it moves multiple squares and if one of the opponent's pieces threaten any of the intermediate squares of its movement. when, say, a bishop moves from f1 to b5, if opponent has a pawn at d5, that pawn can capture the bishop by moving to c4 in the next move.
• otherwise standard chess rules apply.