Hear this out loudPauseTo silence the Nightsong you’ll need the book you can find in the Silent LIbrary. After defeating the justiciar duellists, disarm the trapped bookcases all around you and search the second bookcase on the right as you enter.

Should I let Balthazar take Nightsong?

Letting Balthazar take the Nightsong is generally a bad idea, but it will let you get away without a fight. However, Shadowheart will permanently abandon you if you choose this option. If you want to kill Balthazar, it’s best to do so before entering Shar’s domain.

What happens if Shadowheart kills Nightsong?

Essentially, Shadowheart will be sent down a dark and somewhat evil path by embracing Shar completely upon killing Nightsong. There are still ways of romancing her further on, even if she mentions she has no time for the player, but it is under different and more gruesome consequences.