[@Veridis Quo] Ok so to me, demonic summoning falls straight into full on magic ability and not superhuman. Not to mention all that stuff happening would warp you psychologically forever because demons are generally described as extremely violent and uncaring. 
Hearing those voices at all times would drive you bonkers.

So how about demonic metamorphosis instead? No demon summoning or talking to the dead/demons but you DO get their horrible traits. Fangs, eyes, grotesque wings, long scaly arms with rending claws, hunched back with goat legs, etc.