The pawn. It is a straightforward one:
You move it one square at a time always forward, except when it sits on the second row, in that case you are allowed to move two swares in an only move. It does not captures in the same way it moves, it captures diagonally (only pieces at one square distance)
And wait, I've come up with yet another absurd rule, if it's on the fifth row and a pawn from an adjacent column moves two squares, it can capture as if the move was only a one square push.
Wait, this piece is a mess, it would never be used on a chess game with such complicated rules
The double move rule was added because chess games started too slowly. The en passant capture rule was added so that locking up the position wouldn't be so common.
The Blob. It grows bigger every move and can swallow pieces.