What is Fog of War chess?
Fog of War chess, sometimes called Dark chess, is different from standard chess in two major ways:
Squares you cannot move a piece to are covered in fog.
This is where the variant gets its name. Each player sees a different version of the board, each seeing only their own pieces, and the squares their pieces can move to. If you can't move any of your pieces to a square, you can't see what's there!
Take this position for example:
The grey and dark green squares are squares that White can't move any pieces to. Any of these 'fog' squares could have a black piece on it, and White wouldn't know!
White can see the black knight because the dark square bishop is able to capture it. Black will have no way of knowing his knight is under attack unless he has a piece that can attack White's bishop.
In Fog of War chess you have to use your intuition and also your knowledge of chess in order to predict where your opponent will move his pieces, because most of the time they'll be hidden in the fog!
The goal is not to checkmate the king, but to capture it.
You can capture the king? Yes, that's right, the unattainable dream of many chess players can finally be done in this variant!
In Fog of War chess players will not be notified when the king is in check, and so will not be forced to move out of check. This means that if your opponent does not realize they are in check and do not move out of check, you can capture their king and win the game!
In the above position it is Black's turn! White had no way to know that his king was in check because he couldn't see the queen with any of his pieces. White did not move his king, and now black get's to capture the king and win the game!
Additional rules to be aware of:
Since the king can move into check without knowing it as a part of this game, this also means that the king can castle while in check, and can castle through, or into check!
En passant capture is allowed. The threatened pawn and the square it moved through are both visible to the capturing player, but only until the end of the turn.