Mate in 8

The mainplan 1.Sc4? threat 2.Sd8 is refuted by 1...cxd6!, so the white pawn must be eliminated.

1.Sg2 (threat 2.Sf4) Se2
2.Sh4 (threat 3.Bxf5) Sd4 (if 2..d4 3.Bc4)
3.Sd8+ Kxd6 (if 3...Ke5 4.Sg6+ Kxd6 5.Sf7+ Ke6 6.Sf4)

Now the knights backtrack.

4.Sf7+ Ke6
5.Sg2 (threat 6.Sf4) Se2
6.Se3 (threat 7.Bxf5) Sd4
7.Sc4 (threat 8.Sd8) dxc4
8.Bxc4