He wants you to sink a ship and then tries to betray you. Has his sister try to kill you no less.
So I dead thralled the sister, travelled to his hide-out and used invisibility and sneak to have the sister slaughter the entire gang in that cave, ressurecting her when needed. I used fear to save the Argonian brother for last.
When he killed her I revealed myself to him, turned into a vampire lord and killed him.

I pretended him being very horrified by the whole ordeal, releaved and sad at the final death of his sister and finally scared out his his wits when his life was drained by a monster who told him that it was a really, really bad idea to have tried to doublecross him.

I enjoy using vampire lord on certain enemies. For the last few playthroughs it was the final fate of Madanach the Forsworn king as well.