Moved to the Story & Character subforum where this thread seems to fit better.
And I'll admit, as much as I enjoyed piecing together what I could of the story of the Shadow-Cursed Lands, I was still left pretty puzzled by the end of it. I was hoping that knowing what I know now, it'll make more sense next playthrough!
But I had started to wonder ...
Whether the Thorms were always a Sharran family, hiding amongst the Selunites and undermining them from within. That does seem like something the Sharrans would do. My tentative theory is that Ketheric was brought up a Sharran, but Melodia was a genuine Selunite and Ketheric really loved her, and Isobel followed in her mother's footsteps. I'm not sure whether on this theory Ketheric was always secretly Sharran, or whether he might have wavered and therefore potentially Shar had something to do with Melodia and/or Isobel's death as a punishment for him.
I'm not sure how, even if that is true, it helps with anything else. It might explain why Ketheric didn't turn to Myrkul straight away, given he hadn't actually turned to Shar after Melodia and Isobel's death if he'd always been a Sharran but now no longer felt as much reason to hide it. Aylin's presence was what kept him alive, but I'm not sure what for.
I also was a bit puzzled about what the Shadow Curse was for. Was it Shar cursing the area for Ketheric's betrayal (which would mean that had come much earlier than makes much sense for Myrkul and the other two's Absolute plot), or was her curse because of the defeat of Ketheric, but in that case why did it seem that Ketheric's family were cursed as well as everyone else?
In short, I agree this could all do with a bit more clarification, if anyone is really meant to understand it in a single playthrough!