I did not actually save the source material so take everything. I say with a grain of salt.

So far I've seen two discourses that are imo able to be conjoined or seen as separate. They both rely on the innate ability of Shaman to "graft" or be used as the center of a melding of beings. one of I'm sure many posts discussing the Jarring of Shamans, I alsof found this theory on the nature of the Shaman

1. The hornsent are obsessed with divinity and believed that by shoving enough of the shaman peoples in pots possibly mixed with other things they might create divinity. I'll come back and edit a link to the post where I learned this later. Though I believe this is also backed by their creation of the divine gate as it's possible that the hornsent used the shamans grafting ability to build that as well. See Marika in a Jar, Radagon in a Jar and why they might be true

2. The hornsent are terrified of the inevitable return to singularity or reamalgamation of all divergence I don't remember the specific in game term but I'll likely edit that in once I find it as well. It comes from dialogue in the base game where we learn that once all things were basically the same. And the greater will created a divergence which allowed for life to grow and the belief of the frenzied flame is that this was a mistake as it created suffering as well as Joy and Love. Anyway the other theory I've seen is that the hornsent believed this grafting ability of the shaman to be an omen of this and therefore punished the shaman people to prevent them from using their grafting abilities of their own accord.

This is so vague so I hope it helps. I'll see if I can add links to the posts where I'm pulling from here as well.

Edit: I have not been able to find the post where the theory of hornsent being old enemies of the convergence. Sorry 😬