The Anchor-Vessel Priesthood was never one church. It was a scattering of small, independent orders, each guarding the same craft: the rite that seated a daemon's presence into a prepared vessel and held it on the material plane. Other cults kept their gods. These orders kept the gods present. A harvest-priest of Aedorath could perform his rite for fifty years and never learn to bind his goddess into the vessel that let her attend it. For that he sent to the anchor-vessel order nearest him, and he paid.
Payment is the thing to understand about them. The orders grew no grain, fought no wars, ruled no cities, and foretold nothing. They sold a single service, and every other priesthood in the Golden Age was a customer. They alone worshipped Krathokh, the daemon of the binding rite itself rather than of any thing the rite produced, and their doctrine was frank about what that made them: the load-bearing course of stone under the whole pantheon. The other cults treated them with the wary courtesy owed to people you cannot do without and would rather not think about.
Pay the binder first. Pray after. — counsel given to new cults across the Golden Age
A binder took thirty to forty years to train. The rite could not be written down and then performed by a literate stranger; it had to be carried in a living practitioner who had been seated through it himself, the way a key is cut to one lock. That single fact decided the order's fate. The orders were few, scattered across cultures that otherwise shared nothing, and slow past any hope of quick replacement, and when the God War came every faction worked out at once that its rivals' gods could be reached through these people. What followed was not a battle but a harvest. Sanctuaries were stormed for the binders inside them, who were killed where they could not be carried off and carried off where they could. Inside a single decade the orders were operating at fractional strength across the world, and the bindings they had not renewed began, quietly, to slip.
The collapse is described in full from Krathokh's side; what the orders left behind is the residue worth a closer look. The rite did not vanish. Complete texts of it survive in three or four ruined seats, legible to anyone who can read the dead liturgical hands, and useless to all of them, because the words were only ever half the craft and the other half died inside the last binders. Mernath, much later, assembled the memory of the method into something that could be studied. Nobody has performed it at scale since. A GM can hand a party a perfect, intact manual for seating a god into a vessel, and let them discover the long way that they are holding the score to music no one alive can play.