I would appreciate some assistance in learning about a particular topic.
If my interpretation is correct demons from The Bible and symbiotes from Marvel and vampires from Buffy/other IPs are all just different ways of talking about the same thing.
Could you please comment with or link to any relevant information you have on possession/symbiosis?
As far as I have been able to figure out in my research, what we call demons are the discarnate spirits of Nephilim that have died, and who are unable to either reincarnate or move on to the next realm. They are trapped here on Earth, feeding on negative human energy, so there is something vampiric about that.
I would refer you to sources on this but there are none. Rather, the evidence is scattered in many pieces and no researcher has put it together the right way. It's a very long story but if you have any questions I'd be happy to answer.