Here’s a simple challenge.
Wtih black to cooperate and move first, and white to checkmate, checkmate black in 6 plies (half-moves.) Only one illegal move may be made, and white makes only legal moves.
State what the illegal move is.
You must also prove that the move is illegal, i.e. retrograde analysis is nessacary to solve this puzzle.
Bonus Task (Optional, for those who would like to do this!): Construct a proof game that leads to the given position as additional proof.
Bonus Bonus Task (Super-Optional!): Find the shortest way possible to the current position! If you want to switch the colors to save half a move, you may. Rearranging the pieces is also allowed if it leads to a faster way to a similar position.