So anyone else wants to deal with the Trios dilemma the way I suggest it? Have Yss form a new avatar out of the Nagas and get all that sweet worship by subsuming the faith?
My first thought is that this is dishonest, and that can lead to headaches when you try to promote and maintain that on a large scale. That said. I don't have a better idea really, so I don't want to dismiss any out of hand.
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On further thought, the only alternative that comes to mind is to talk with Messenger, then together (loaded with social buffs, and An cast on messenger), explain the truth to a couple of priests (or maybe Teleport in ones from distant cities too for a whole bunch). Their real faith may have been to a false god, but the apart from these trappings, the core of teachings, of self improvement (or whatever it was), is true. And is a path worth walking, one of wisdom.
You know, generally comforting things to help get them through the shock. Then talk with them to see how we can get them to educate their faith, properly, on such matters, maybe in groups. And also add things (like, government type programmes but run by the church, but appropriate for aswah context) that focus on self improvement that they can partake in as a community, so they can hold to them tighter in the face of having massive parts of their faith (well, all of it), stripped away.
Oh. And ask Yarel for help. He might be a martial type, but he's all about truth and the such. He'll totes be able to throw in a few ideas for this sort of thing.
It might be helpful to know Messenger's current relationship with other priests in the church for planning this. And to find out what their general knowledge planes and knowledge religion is like. If there are any high outliers, I wonder how they took the realisation their God is false.