As Tav there are two ways to save Jaheira while killing the Nightsong;
- Lore-accurate way - You'd have to reload a save prior to Marcus revealing himself and intentionally let him abduct Isobel while fighting him. This way your character would still maintain their neutrality by simply failing to protect her.
- Lore-inaccurate way - Reload a save prior to entering the Shadowfell and just go kill Isobel. Unfortunately your character will be considered a traitor within the story.
