I read that if after the boss fight at the top of moonrise tower you descend with a summon (an elemental in my case) the cutscene about Nighsong and Isobel joining your camp doesn't start because the other npc have a fear/bad reaction towards the party. I copleted act 2 with Nighsong and Isobel alive but didn't get that cutscene.
If that is a real but please fix ASAP and also put an NPC at the camp in act 3 to "summon" Isobel and Nightsong just to fix the save for people who are already ahead like me. You could make a dialog line with the undead for respecs to just recover Isobel and Nightsong, or just let people teleport back to act 2 (a cheat with old portals unlock could fix it maybe).
I surely cannot complete the Shadowsong story; probably not even main quest (not sure about this).
Help please I'm trying to find a workaround: cheat, console, mod, official patch (much better) to fix it.
Edit:
Killing a specific mage fixed all and unlocked all the dialog Lines that Ware clearly for the end of act 2.
