As our Harvey Randall wrote, we’re all still playing Baldur’s Gate 3 more than a year after its release because there’s so much to find in it. Gale can write a sad resignation letter if you ignore him long enough, and there’s a game over just for people who go out of their way to lose a plot-critical item. When I was playing I triggered an ambush by Astarion’s vampire-spawn siblings by long-resting repeatedly in the Lower City, something that apparently a lot of other players didn’t experience. Even I didn’t notice what happens if you let Astarion get vamp-napped, however.
As documented on the Baldur’s Gate 3 subreddit by Soft_Stage_446, Astarion gets taken to his master’s torture kennel to be punished by a skeletal interrogator called Godey. An entire scene you’d otherwise never see plays out from Astarion’s point of view, whether or not you chose to play him as your origin character. And then you’ve got the opportunity to escape as Astarion, or switch over to the rest of your party and launch a rescue attempt. There’s even dialogue for various companions’ reaction to Astarion’s abduction.
It’s not a sequence you’re likely to see by accident, even if you’re a habitual long-rester like me. The vampire spawn aren’t a challenging fight, so you have to go out of your way to let them down Astarion and then make off with him. And you have to have him in your party when the attack happens. If you don’t he’ll still participate, but not helpfully. (When tested, he spent 15 rounds throwing fireworks instead of attacking.) And if defeated, in this case he’ll simply die.
In terms of rarity, this is right up there with the conversation you can only experience by choosing to be the Dark Urge, and trying to collect your inheritance from the Counting House. Or the ending you can only unlock by playing as evil Lae’zel, and doing it badly. Only 34 players had found that when we wrote about it, but those 34 people had a great story to tell.