!!!!Massive spoilers for this quest ahead.... obviously.
!!!!
The eventual solution is quite a lot simpler than my long ramblings I feel are necessary to explain the problem... but please bear with me here..
I ran into this one last night on my second play-through.
On the first play-through a couple of weeks ago, everything worked as expected: I promised the spawns I'd set them free, they told me about the staff (Woe), I killed Cazador rather quickly, staff "dropped", all cut-scenes played as they should (talked Astarion out of doing the ritual himself), spawn went into the Underdark and the Gur were kinda mad at me for setting free 7000 vampire-spawns.
Second play-through was a bit of a head-scratcher. What happened was that the staff did "drop" during Astarion's cut-scene (when he drags Cazador out of the sarcophagus), but all the staff-related cut-scenes and narrator-lines that should follow were missing. Also the "ritual-locked" spawns only ran from the chamber at ludicrous speed after the cut-scenes happened, and their dialogue-sequence never triggered. I also did *not* automatically receive the staff after all the dialogue had finished. The result was that on our way out of the dungeon, my party automatically told the Gur that their kids had been turned and that was that.
I thought I'd made a mistake/failed to use a crucial question/response during the initial conversations with the spawns before the fight, so I went back to an older save and re-did those two conversations "correctly" (intention being to have them tell me about the staff and then to promise to get it and free them). That didn't help either.
I tried both letting the dialogue play that occurs when you first approach Cazador (resulting in Astarion getting sucked into the ritual) and to attack before any of that dialogue triggers. Didn't make a difference.
It also seemed to me that the transition from the cut-scene with Astarion going nuts on Cazador after the fight looked a bit off, like something was missing here. So I kept trying.
I was at my wits' end and started to grasp at straws. It seemed a bit silly to me, but there was one obvious difference in my behavior between my first play-through when everything worked as expected and this glitch-y behavior I was experiencing now: I never used the Daylight-spell during the fight on my first play-through. I didn't expect this to work, but since I'd always used Daylight in all my attempts in the second play-through, here's what I tried:
When I used Daylight before, I always cast it as a sphere, not on an item. I usually put the sphere right in the middle of the room, near where Cazador starts out when you first enter. I noticed that after the fight, I got quite a few log entries re Sunlight Allergy (or whatever it's called) for the ritual-locked spawns as they sprinted out of the place. They run super-fast, and the reports flash by quickly, so it's easy to miss this.
Since I know of no way to simply switch off that Daylight sphere (it's not a concentration spell I think), I went ahead and cast Daylight on Karlach's melee-weapon this time before the fight started. When the fight was coming to an end (but before it *actually* ended), I switched Karlach to a different melee-weapon, effectively switching off Daylight. I even switched Shadowheart from Blood of Lathander to her bow, just to also get rid of the Lathander's Light-effect.
Sure enough, that did the trick. This time, the spawns didn't simply run out of the place after Cazador got murdered, but approached the party and started their conversation with them. I also got all the staff-related scenes/lines of narration and was able to free the spawns.
I can't 100% prove that the Daylight sphere is what caused this, but since I ran this fight way too many times to try out all sorts of things and the Daylight-stuff described above was the only thing that made a difference and let the dialogue run as expected, I think it's safe to say that Daylight was to blame here. Perhaps it would be an option to have the game extinguish that Daylight effect automatically once the fight is done so that the dialogue-scenes can happen without this "glitch"?
Or perhaps this is intentional to punish players for ignoring a fundamental aspect of vampire-lore?
S.
