I've casted Polymorph on Isobel, because it was said to render her unconscious and not to kill her. So I've thought I could render her useless with a Polymorph spell for the time being.

As the last Harpner died, the cutscene startet with Markus flying away (with a sheep in his hands, that was funny), and the shield of Last Light breaking away. The cutscene was immediately interrupted by the start of the fight with the shadow-cursed harpners. After the fight, Markus and Isobel casually remained in her chamber, with the quest not progressing.

After I resolved the Nightsongs quest, with "Isobel dying to the shadow curse".

During my first playthrough, I've made a lot of conflicting choices, because I was exploring the game thoroughly. I've promised to free Halsin, helped him in his fight and told him to meet me at the grove, yet I've sided with the Absolute, fought Kagha, and finally stole the Idol of Sylvanus.

The whole thing with the druid grove got bugged as hell. Before the Absolute even attacked, the whole grove was slain, and the curse got a grip of it. Only after a rest or two, the absolute troops entered the grove, with Minthara being sad that I've started without them. After another rest, the barrel-hidden goblins appeared infront of the grove.

Only after I've entered the shadow-cursed lands did the quest resolve, after being stuck on "Meet Halsin at the grove".