I see Diabolos being some big Voidsent demon rather than a Primal. Similar to how Siren isn't a Primal, but rather a very powerful monster. I think it be cool to have him but I don't remember if there is a darkness element which he'd be perfect for. If not then I kinda just see him as some super demon to face off against in an environment like Tam-Tara or Haukke Manor Extreme.