A pure Skyrim vs thread.
So, a vampire has performed the Black Sacrament on Harkon. . . the Dark Brotherhood has recently recruited a level 35 Dragonborn. Assume the Dragonborn has any class. (Not all classes, any class). The DarkBrotherhood has fewer numbers, no Deathhounds, etcetera.
So I'm wondering, is there any way that the Dark Brotherhood can assassinate Lord Harkon? If not, assume the Dragonborn has befriended Serana, and she will except the DarkBrotherhood's help in defeating Harkon. (In this situation, the Dawn Guard never trusted her, so both Dragonborn and Serana were kicked out).