There have been more than a few theories about Shadowheart, and some attempts to work out the timeline. Ketheric Thorm was a Selunite cleric or paladin, who suffered a great loss that led him to worship Shar, for some time he worked to subvert the Selunite temple, probably the one at Moonrise. The loss he suffered might have been the death of his daughter, but last time I checked, they'd expunged the references to that from the EA, it still led to speculation that Shadowheart was somehow connected to Thorm's daughter. Eventually Thorm revealed himself, betraying the village, and starting the great conflict you still find artifacts of around. Also note that the village says children have been going missing, probably a result of Thorm. It's unclear how it happened but Halsin seems to view the victory over the Justiciars as either incomplete or pyrrhic, and it is probably the cause of the curse around Moonrise. We also find evidence of a Hellish host killing the remaining Justiciars in Grymforge. I try to avoid datamined spoilers, but the Nightsong is also probably pertinent to this plot, as well as the Sharran Temple.
I think Shadowheart was proabably kidnapped as an street urchin in BG, I can't quite work it out if she's directly involved with Thorm and his Justiciars. Though I did have a fanciful theory about Shadowheart's personality reverting to a kinder gentler version of herself because most of her memories as a Sharran were removed.
Okay, and meanwhile, as all of this is happening, where is Selune? She (a greater deity) just sits there and allows her faithful to be "kidnapped" and "brainwashed" by Shar, her arch enemy no less, and is totally cool with it?
This is Larian BS that completely goes against well-established FR lore, and a good example of why I consider Larian's writers to be ridiculous jokes. A good writer accepts the constraints within which they're expected to write, and they still end up creating something awesome even while honoring those constraints.
