I agree Bleoberis, if you are building an NPC, I'd just give the warlock a few powers that are both warlock-y and demonic, made up whole cloth or based upon existing powers from the warlock and other classes.
I've actually been working on a primordial pact warlock, but haven't gotten as far as I'd like. But it focuses mainly on heavy damage, elemental attacks, lots of pushing, dazing, and knocking prone, to conjure images of the raw force and chaos of the Elemental Chaos. Maybe something flavored like that would make a good "demonic" warlock?
Or just look at some of the powers demons have in 4e, and tune them up or down, and give them to your NPC.