A [Human] [Old Ones Warlock] who is possessed by a whole pantheon

Instead of just him signing a "pact" with one entity, he's a normal nice enough guy who happens to be like a conduit for just about every horrible Lovecraftian thing you could imagine. Like if you choose Witchbolt, he isn't a spellcaster casting 'Witchbolt'. Instead, he is possessed by "Lmnok'th The Living Light", who comes out of his mouth and ears to antagonize whoever is threatening its 'host'. He isn't "casting" Eldritch Blast", an eye just opens in the middle of his hand, which shoots its psychic arcane mucus at people. He's not "casting" invisibility, Nyrat'hupu the Silent God Of Screams is just covering his entire body, and is thus hiding him from human gaze.

That kind of thing. Get creative with it.