Welcome,
I'd like to pool the weird bugs you've encountered, that broke quests for you.
Remember, you can use spoilers.
nameofspoiler
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Resolve the abduction
Act: II Probable cause of bug: Polymorph on abduction victim
Description of what I did
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.
Description of what happened
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.
How did the quest resolve
After I resolved the Nightsongs quest, with "Isobel dying to the shadow curse".
Rescue the Druid Halsin
Act: I Probable cause of bug: Making conflicting choices?
Description of what I did
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.
Description of what happened
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.
How did the quest resolve?
Only after I've entered the shadow-cursed lands did the quest resolve, after being stuck on "Meet Halsin at the grove".
Act 2 - Too many weird issues to count with Halsin in act 2. Thankfully I was able to talk to him at camp, get him to go meet me at the Inn and then do the Defend The Portal thing. Then he disappeared for a while, had nothing to talk about at my camp, and the "Show on map" button for his quest didn't go anywhere. Thankfully it did resolve itself later at the end of the act, I think it might have been caused because I did the hide & seek with the cursed tiefling boy "too early" in act 2?
Early act 3 - The booby-trapped toys quest, after getting the Fist officials to realize the toys are trapped, you get a quest update to go confront the toymaker. Weirdly the only options are kill him or let him leave, not turn him in? I let him leave, he vanished and the quest never updated so I still have it.
Late act 3 - All quests related to Kithrak Voss and the Orphic hammer - I spoke to Voss at the start of act 3, then to Raphael, then Voss again. Many hours later, stole the hammer, went to meet up with Voss in the sewers. He would talk to me, but he just said "Follow me to the tap room", and then he never left the spot. Tried talking to him on all characters, going to camp, reloading, etc. etc. I think this one might have broken him because I normally have at least one summon out (celestial or elemental from Shadowheart, Wyll or Gale) and NPCs go insane if you have a summon they interpret as hostile. I was able to progress the quest but maybe this interrupted him somehow. Anyhow it would have cost me so many hours to go back to a save before that by the time I troubleshot everything that I just gave up. You can still break Orpheus free at the end and get Voss's help, but you never get the greatsword (which is a shame because Baldur's Giantslayer literally does not fucking work on the ONLY two mobs in the game that qualify for its bonus damage after the point where you receive it).