Can someone shine more light on what was going on in Rime of the Frostmaiden in so far as it is relevant for the netherese aspect of BG3? I think we should limit the use of spoiler tags to things that cannot be learned ingame.
Shadowheart being a weapon would tie in with the brainwashing theory and the strange magic effects she experiences sometimes. Especially in proximity to things related to Selune. Why would the Sharrans decide to force recruit a Selunite for a mission?
Rime of the Frostmaiden: there is a Netherese floating city that crashed centuries ago (when Netheril was destroyed) over Icewind Dale that is now under the ice. I don't think it has much to do with BG3 personally, as it pre-date the Shadovar (the Netherese who use shadow magic).
Back to Shadowheart
I very much doubt she's a brainwashed Selûnite thanks to datamining. She likes darkness and the Underdark way too much. I'm starting to think she has Shadar-Kai ancestry actually.
She doesn't experience anything near related Selûne things (no reaction at the untouched shrine in the Owlbear cave), only things that shows/mention the passage of the Dark Justiciars. People aren't paying attention to the details here.
Her personal quest at the end of the EA is called Chosen of Shar and Nigthsong calls Ketheric a Chosen of Shar. Ketheric was the leader of those Dark Justiciars. It all look connected to me...and not really relevant to the Absolute. Unless the Absolute and Ketheric are acquainted...
