I really don't see why we would need to be hostless, unless we are being scanned for possession, in which case a manifesting demon would have to be detectable too...
Because Mongoose is mortal. Because he can be captured and we may need to succeed. Because any of a thousand things could happen which deprives us of our host or even our whole cult, and because we can't expect the universe to give us anything we need, no matter how much we need it.
We would not associate with our host's former acquaintances unless pressed, would typically choose hosts who lacked social support, and have done perfectly well at mimicking our hosts thus far.
What makes you think it would be our choice, would we then scan our potential host for however long it takes to determine they don't have any relationships, and would we then toss aside our cultish desires for the sake of the masquerade.
Mongoose being literally human really doesn't seem to be an issue to me.
There's
another factor that's harder to put into words. Basically, if we are actually a horrible monster and just inhabit a human, if forced out of or leaving the human body we become "killable," not to mention less chilling and fearful. To explain the latter...have you ever seen Resident Evil 4? Were you more afraid of the human-but-supernatural Wesker or those weird zombies with the deadly tentacle things?
In combat, it would be less about potency, though form is a factor and human could be improved upon, and more about varied abilities.
Which ones would we be unable to add to a supernatural human form?
There are many physical abilities outside of combat,
Which ones would we be unable to add to a supernatural human form?
we already have cultists if we need human abilities,
So, we're going to become dependent on a resource we can't guarantee the longevity or loyalty of?
but if we need to pull something along a narrow drainage pipe without being detected,
...Why? And snakes aren't too good at that...
or stretch up to reach something two storeys above us,
How would a snake thing do that?
or fly,
How would a snake thing do that, and can't we do that in gaseous form?
or have folk turn into stone if they look at us, or swim indefinitely(Actually, I expect that water currently represents a significant obstruction to our gaseous form, an aquatic form would probably be ideal.), or fight armed opponents when unarmed(Which really requires some form hardened body part, which is fine for a claw, but harden your whole arm and it impacts your mobility...),
Why couldn't we add those to the supernatural human? For the last, such a human would do better than a snake.
or hide where a human couldn't,
Gaseous form.
or be worn as clothing,
Why?
or be identified as a peaceful woodland creature with an antidote to all toxins...
Why, and what's wrong with being identified as a hermit with an antidote to all toxins?
As for social engagements, If our true form becomes known, and it is human, then others might suppose that we really are human, and if they believe that we chose to be human, then what does that say to dwarves? Or elves? Or mermaids or orcs or goblins or pixies or minotaurs or 'obbits or humans of a different age, gender, racial group, height, hair or eye colour, build, proportions or anything else that might catch their eye and ire? When we could choose anything, being just like everyone else is a wasted opportunity. I vote sea serpent!
Why would this matter? And, for that matter, did you note that I was also suggesting forms which could easily pass for human in proper clothing?
Generally speaking, critters either breath water or air. There are exceptions,
Cutaneous respiration and modifications. Heck, even some turtles can breathe underwater.
Well, we could just cheat and have one of those serpents that can fly just because.
Couldn't we be a human/oid that flies just because?
I think that you can qualify as a sea serpent with a diameter of only about four metres, you could get around on land with that.
Oh, yes, great for dragging stuff down narrow pipes and hiding places humans can't...
Alright, I am going to put as a thin tall man with slight reptilian characteristics. For example, he has vertical pupils, and his skin look scaly up close(said skin is also tougher then basic human leather). There's this mild thing that just a little bit wrong about him that would ennerve anyone quickly, but demon ancestry is common enough that people wont start burning you at the stake just for looking a bit "off".
When being chased by a tall, hissing man with a larger amount of toughness then any normal human, I'd personally be scared.
Well, we can pass for semi-normal, so this is good enough for me.
Can we go with an amorphous physical form? One that can move underwater or on land and is slightly blob like so it's hard to hurt with weapons. We could also form arms, legs and other stuff like that if we needed to but normally we would just get around my slithering around like a little fast moving amorphous eel. Oh and with a form lik that we could slither into peoples' mouths in combat to possess them or eat them from the inside out, almost like a horror movie.
Seems a bit OP...