Heroes of the North
Durgan the Bonegineer is clearly brilliant, and his undead monstrosities have plagued the Triad more than once. He is almost certainly dead, slain by Cyndas, but it is uncertain what that means for a Death Knight. As an ally, he would provide a small army of stitched together horrors.
Recruiting the Bonegineer would be an odd undertaking. Frankly, it would involve less convincing and more coercion. The downside is that the Bonegineer would be unreliable without a tight leash.