Helpmate in 5

Given that Black’s king and a rook are on their initial squares, 0-0-0 is a strong likelihood. Post-castling, a potential mate with White’s knight on d6 is easy to envision; we just need to block the flight squares c7 and b8. The knight on d5 is already poised to cover c7, but taking care of b8 requires a more intricate plan. Nevertheless, a clear solution emerges: promote the b2 pawn to a rook (of course, we don't want to unnecessarily check the wK by queening the pawn), clear the b-file, and slide in Rb1-b8. It turns out a unique five-move sequence realises all of this:

1.0-0-0 Ka2 2.b1=R Ne4 3.Nc7+ c4 4.bxc3 ep.+ Ka3 5.Rb8 Nd6#

The sequence remarkably fulfils the famed Valladao Task, combining all three special moves of chess: castling, promotion, and en passant.