This is Hunter Chess. You can en passant with the sergeants in this variant, and let me explain how the pieces move: 
The Sergeant moves like this. Orange means move or capture.
This is the Cannon's movement. It moves in one diagonal and captures in the other. Red means capture and blue means move.
This is the Fortress movement. The Fortress moves like this, which is extremely OP. We might nerf the Fortress sometime soon.
a newer topic, post your variant ideas here