I think I'll take a form resembling thick black smoke. Able to flow through the tiniest crevice, I can get into anything, and if I spread my body through a human body, I can practically control it. Spread myself so thin that no one can see me, or become as dense as a concrete wall. So yes, sort of like the Smoke Monster from Lost, but also like the Vashta Narada from Doctor Who with a dash of Minecraft Enderman.