Fredrik Svanberg
First Post
Demons and other outsiders could be natives to other planets or deep space instead of planes. Summoning rituals bring them to the summoner's planet, for a period of time. That's how it works in Call of Cthulhu, after all.
Vampires, werewolves and zombies can all be given "scientific" explanations based on viruses, genetic disorders, nerve toxins, etc.
If you make "ghosts" and "magic," the result of psychic manifestation and sufficiently explain psychic power in pseudo-scientific terms those will work too.
Demons on the other hand...not sure.
In the folklore tradition of many countries, Hell was a place that you could simply travel to, not so much another dimension or plane. As a matter of fact, I never really loved the term plane as I'm unsure medieval Europeans would've used that word to describe places like Arcadia/Faerie, Asgard or Purgatory.