You make good points overall but you misunderstood what I was saying about their power compared to the druid grove. What I was saying is that we don't know how impressive their ability to massacre the druid grove actually is because we don't get a good sense of how powerful druids in this setting generally are. We see them being intimidating to a bunch of ragged refugees, but that doesn't give us as players a baseline. I can guess how powerful the average druid is expected to be, but I don't have any good comparisons aside from the already vague fighting power of the cult. So we have one fighting force we lack good context for able to massacre another fighting force we lack good context for. Your point about the flaming fists is a good one though, I'd forgotten about thim. Though it gets even muddier with your point that Halsin could have easily broken free when he wanted to, which makes judging the strength of either group even harder.
Also to your point about them controlling the region fully, it's a region without a lot there. There's a village they destroyed, a tollhouse, a druid grove. It seems like some random backwood of indeterminate size that doesn't have anything of worth to the world beyond. We don't get a sense of scale as to how big this region actually is meant to be, and in general the sense I get is that this region is just a place people go through to get to other, more important places. Sure, they have to start somewhere, and if it was clear that they achieved all this in a really short space of time that could be impressive, but we don't even know how long they've been at this.
This isn't even just a problem with the cult path, but with the game as a whole, it seems allergic to giving you concrete context for the setting as a whole. That sort of thing is fine for a game like darksouls, where ferreting out information is part of the game and not knowing the full context won't generally take away your enjoyment of the game as presented. In a cRPG though, having an investment and understanding of the setting is vital to telling a good story. I'm all for players digging for more lore, but the game still needs to give us the baseline information we need to let us care about the given plot points.
