Alternatively, you could make a music instrument implement for the warlock (much like the blade implement, you chose the instrument [I might regret that if it became official and big instruments were picked so you get cover]), give it a basic ability (as long as you use your action to play the instrument, one target that can hear you is charmed), and a couple of invocations (use a warlock slot to create an effect similar to antipathy or sympathy which lasts as long as you play seems like a good high level one, others could let you cast support spells). You have charisma, music, and can (magically) do a bunch of sneaky stuff.