The plot thickens.

https://www.gamespot.com/articles/baldurs-gate-3-nightsong-kill-or-save-guide/1100-6516519/
Consequences of killing Nightsong:

Deaths

Isobel - Her powers as a Selunite Cleric will not be enough, and Last Light will get ransacked by cursed forces.
Jaheira - This returning character from previous games and potential companion will die offscreen.
Dammon - The blacksmith will also end up dead, preventing you from giving more Infernal Iron for Karlach's romance and quest.
Art Cullagh - The sleeping man will also get killed, potentially locking you out of Halsin's recruitment and Bear Form romance.


https://www.gamerguides.com/baldurs...in-bg3-location--puzzles-choices-rewards
Consequences of killing Nightsong:

If you kill the Nightsong, then you complete the tasks that Shar was whispering to Shadowheart the entire time you were in the Gauntlet of Shar. Doing so, you will be awarded by Shar as she lifts the curse, allowing you to kill Kesaric Thorm, as he has now become mortal. In turn, Shar will make Shadowheart a Dark Justiciar. On the next long rest, Shadowheart will even change her hairstyle to match those depicted on Shar statues (bless her).