Needless to say Inside the Tain depicts a very twisted realm. It makes us wonder why the Smiths of Muirthmne became enraptured by this world and formed the spider-cult. As for the Fetch and their relation to it, I think you are slightly mistaken. One other artifact that Soulblighter possessed was the Tramist's Mirror. As he used it to make Shiver coropeal once again.
I would think that the Fetch, as seen by their death sequence, are spirits that become lost when killed in this world. They are from a realm where spirits flow freely, such as the ether from the other side of the Tramist's Mirror. I just see it hard for them to co-exist with the spiders when they are the best destroyers of them within the Tain. So they seek a way to return to their world so that they can roam freely once again and not be lost forever in this world.
One area I need clarified is how the doorway between the spider world and the Tain works. The comic passage is very vague in that manner. We know that whoever is trapped within the Tain is taken to this world/place, and that there was an escape route or backdoor of sorts. But the exact relationship between them is very hard to discern. If you have any ideas on this please let me know.
Sincerely,
Pimpy™
