perhaps i am simple, but I think the most terrifying creature is one that looks like me

I mean of course possessed humans, even children. Ones that make you like them, or more perhaps... comfort you when you are sad even.....all the while having nefarious motives *evil grin*
Beyond that, the golem area could be expanded as well -- if there has been a way to have a soul inside of stone, there is a way to create souls inside of other normally inanimate or sentient beings.... that could be creepy if you really think about it...
All that said, i am just starting to read the books so feel free to ignore me -- i just know when i was young, one of the biggest things that scared me were the things that SHOULD be normal, but werent --- a smiling face being a potential kidnapper --- etc. The biggest devils should have beautiful faces after all, its how they draw you in

That and it, in theory anyway, should be harder to kill things that look sentient, non?
What about a type of humanoid creature that uses possession as their primary form of magic (life instead of blood magic kinda) -- soul leeches, energy vampires -- selfish pleasure driven creatures that see power from the possession of other creatures - a more old, ancient primal magic than the heady lythium enhanced magics popular now? I wouldnt mind some male driven forms either ( ok, call me a girl

) like the Desire Demons i guess but looking more like a cross of an elf and Ares from Xena kind of character ...
bet ya wish you hadn't asked now