You only want the "Power" of the Warlock or are you more interested in the role-playing aspect? In Roshar, all Radiants are, basically, pseudo-warlocks. They get powers because they make a deal with a strange power, and if they don't do what they are supposed to, they can remove them their powers. It's not the same, of course, but you can give the flavor that you want to your spren and make him a more menacing spren that forces you to do stuff, similar to Szeth's.