You can meet a Selûnite priest in the Shadow Cursed Land. He's looking for Shadowheart. Her magic flares happen when she spot him. She used the magical flares against a Sharran during her flashback training and they also happen when she kiss you after the party (according to the scene context notes). So it's not triggered by Selûne related stuff only. The priest claims he knows what it means, but she has to go an abandoned chapel in BG to learn more because Shar is too strong in the area. I find that a bit suspicious...Selûnite shouldn't have to hide in BG.
Her name might actually be a title: The Shadow Heart. As that is how the Selûne Priest refers to her, although it all look like temporary dialogues.
Both Shar and Selune wants her, foretelling her coming and stuff.
if she doesn't do as Shar wants, you get a super depressive Shadowheart around camp...
Looks like the character design of the suspicious priest was datamined.
