Nightsong is one of the central characters in Act 2. After many struggles, you'll eventually come face to face with her, with the option to spare her life or end it. One of the marvels of Baldur's Gate 3 is every action you take tends to have cascading reactions later on down the line, right or wrong. So, should you kill Nightsong or spare her?
What Happens if you Kill Nightsong in BG3?
Killing Nightsong in her Shadowfell Prison will dramatically affect how the rest of Act 2 will play out, as she is a pivotal figure in the final act. Her death will ensure the deaths of several other characters in the story, and the Shadow Curse will become an even more deadly ailment to maneuver. Many quest opportunities will also close the moment you end her life. Personally, I don't recommend going this route on your first playthrough; I think most players will regret the ending it will spawn.
Killing her will also seriously affect Shadowheart as Nightsong deals specifically with Shadowheart's character story arc. Having her kill Nightsong will cause Shadowheart to fully submit to the will of Shar, and she will become even more of a zealot toward the god's cause than before. You'll also end any possible romance with his decision, so if you were hoping to get to know her better, avoid killing Nightsong like the plague.
What Happens if you Spare Nightsong in BG3?
As mentioned, Nightsong plays a huge role toward the end of Act 2; she directly leads the charge on Moonrise Towers almost immediately after being spared, and you are to join her in the assault. The Harpers will also be alerted, and join the attack on Moonrise alongside her. Nightsong's freedom does lead to her eventual recapture, but she can be saved one last time during the final boss in Act 2. I highly recommend going this route for your first playthrough.
Shadowheart will initially become distraught by her sudden decision to defy the will of Shar. Thankfully, this is just a phase, as after the battle at Moonrise Towers, Nightsong informs Shadowheart of her true origin—the origin she was brainwashed to forget by Shar agents in her youth. Eventually, Shadowheart will renounce Shar and become a follower of Selune; her hair turns white as a symbol of her renouncement of the Lady of Loss, and her outlook on life will become much more positive.
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