What if Shadowheart is also a justicar and this is the very reason part of her memory was wiped so she couldn't be discovered and killed until she finally gets to the nightsong and the absolute? Cause when you discover the helmet, she has the same magic manifestation on her hand that she gets when near the Selune statue in the village. I have this impression Shadowheart's story arc is deeply grounded into the absolute raise to power.
Her mission is related to the mysterious artifacts she has. She's supposed to bring the box back to the Baldur's Gate coven.
Now whatever the Baldur's Gate Shar coven leaders are tadpoled and the mission is related to the Absolute is up in the air...
