I like that this is labeled the final part, marks a good spot to sit and collect thoughts on the arc. I mentioned briefly that I like the way this story is highlighting the merits of conventional investigation, while still working in drama when things go outside the scope of the norm. The cultists being anti-demvir also helps draw out a side of Laermont we rarely see, making the endeavor not only righteous, but personal. As a man who has fought hard for his city (and others), the indignity proffered by these lunatics is unconscionable.
The final fight really felt out of place compared to everything that came before. That's not to say it wasn't enjoyable to read, but I really expected more of an intellectual victory, with Laermont thwarting the villains via planning without the ensuing chaos. Still, nice to see a clear victory for the heroes!
I also agree with a lot of what Fin had to say, but now I have to be a spoil-sport and bring up a pretty significant problem: the portrayal of Solaviskar is wrong on almost every level. Before I get into the why, I will say that it doesn't invalidate this story arc at all, except that I would recommend revising it to give this organization a different name. One possibility that comes to mind is Electos Suos; literally 'His Chosen People,' the 'elec' part makes me think of electricity which seems fitting for their fixation on demvir, and Electos is a name that plays well to dialogue I think. However, any name will do and it's your story. Even my thoughts here are merely that, thoughts. By no means is this me bringing down the hammer.
The thing is, Solaviskar isn't a cult, nor is it (inherently) a hateful religion. The Magister himself is a Solaviskar believer. The name literally means "Only the Viskar" and the primary difference between Solaviskar and Inlustrovis is that Solaviskar believers think only Castus, the Viskar, is a god. The other Vis are not considered gods at all, and are at best akin to the angels in Christianity -- Solaviskar followers would not look to 'the Vis' for guidance but specifically, and solely, the Viskar.
It's similar in a sense to how Judaism and Christianity come from the same roots, but Christians believe Jesus is the son of God while Jews believe Jesus was a prophet or just a man, not the savior. Inlustrovis recognizes the other four Vis as divine, but Solaviskar does not.
Solaviskar is also not a supremacist belief that all other races must be culled -- instead, the laicar believe it is their duty to lead the enlil and velen in the pursuit of taming the dangerous world that followed the Cataclysm. Less "you are beneath me" and more "I know what's best for you." Solaviskar also does not hate the demvir. There's a different moral issue: they believe the demvir were created to serve the laicar.
In the "Predominant Beliefs" thread, I mention that Prophet Deimar is 'prone to violent displays where the wishes of the automatons are concerned.' What I meant by this is that demvir who do not wish to serve the Arcanum make him angry. To Deimar, it is their sole purpose and duty to further the aims of the Viskar, as interpreted by His chosen people, the laicar. He doesn't want them destroyed, he wants them doing 'their job' (no matter what their personal wishes are).
All that being said, I find it not only plausible, but likely that there could be a sect that takes things further, one that treads the more hate-filled road that featured in this arc. I just wouldn't call it Solaviskar.
Sorry for that long spiel. I wanted to kind of highlight the fact that the religions were really intended to have degrees of nuance. Fundamentalists and fanatics can ruin any belief, but the core usually has some form of merit. That's both what makes them so dangerous, since they can use whatever good there is to justify heinous action, and what makes them tragic, since they are often delusional in their extremism.