Great stuff!
Grif, thats the direction I was heading with the spider idea: it would be drawing him in and tempting/trapping him, but it will be equal parts intellectual and physical trapping. The way spirit creatures manifest a body gives it some leeway when it comes to specific form and abilities. Im open to but a bit hesitant to go full human-form flesh-mask style with it, having a monster form lurking under the surface and all that. These Spirits, in general, manifest a physical form by taking over some matter that is fitting/relevant to their purview, so elementals just need their base elements, whereas more abstract ones might need something more specific. For example, the greeks were personifications of human concepts and so liked to use humanoid statues of marble or gold; in general it's tiring to hold and animate a physical form so they dont do it forever, and their available power is directly proportional to how closely their form manages to match their "true" form. Possessing a living creature works but gets complicated; it can last longer but may not allow a whole lot of power through; Saints are people with Angelic partners possessing them, for example. Animal spirits are common enough, and usually take over some pinnacle example of the breed, to the point where some have raised specific bloodlines for that purpose (if Mouse existed he'd be something like that) so Im considering having a super-spider being kept somewhere for that purpose. I was always fascinated by the fact that spiders have no predetermined max size, and simply grow as large as their food supply will support. So in theory a spider could grow to enormous size if it was kept well fed enough. It might not be able to move or support it's own weight eventually (the body mechanics dont scale perfectly), but it could get there. Such a breed would likely be in Australia. The biggest and scariest spiders all seem to be from there, it's where Id expect their King to chill. Besides, the Outback is probably one of the more remote places that lots of Nature spirits spend their time, in the modern world.
As a related question, what are your thoughts on swarm-forms for such creatures? I have both this spider-demon thing, and also a were-ant queen (a human in that case, not a spirit). Ive long said swarm and cloud monsters dont really work well for villains on screen, but I dont know if i have the same issues in print.