This puzzle is difficult... Too difficult for an engine even! Agadmator discusses it in a youtube video (https://www.youtube.com/watch?v=8wCJalNkTEI), so if you want you can also go there.1Nf6+??−5.0...lichessBlunder. Nxe3 was best.OcelotBaneThe winning move, even though the engine considers it a blunder.1.d8=QNf7+Promoting loses to Kf71...Kg71...Kg62.Bh5+Bh5 protects the fork on f7, allowing to promote1...Kh82.d8=Q+Promoting comes with check2Nh5+Kg6All other king moves prevent Kf7, enabling to promote2...Kh73.Bc2+The only king move where promoting is not immediate3Bc2+...The move that is missed by the engine3...Kxh54d8=QNf7+Does not work anymore! Our king will move to f5, such that the bishop can mate the king5Ke6Nxd8+6Kf5e2Prevents Bd1 checkmate7Be4e1=NUnderpromoting to knight to prevent Bf3, checmate8Bd5...Rerouting the bishop to mate on e28...c29Bc4c1=NPrevents mate on e210Bb5Nc611Bxc6Nc7Prevents mate on e812Ba4c413Bd1+Ne214Bxe2+Nf315Bxf3#...Finally, it is mate