Is it Nxb8 -
1)if black takes the queen Q*a7, d7 is checkmate
2) if black takes knight Q*b8, Qa4 is checkmate
3) if black takes the pawn nxd6, Qd7 is checkmate
I cant find any other moves that make sense for black after nxb8 so I guess this is the solution
My recent chess problem.
White to move and mate in 2.