Baldur's Gate III is full of choice. You can pretty much do anything to continue the story, no matter how many NPCs you kill, how evil your decisions are, or which path you choose to take. There are a load of videos online showing all the different ways you can play, but apparently there's a big secret no one knows about.
Speaking during a Twitch stream where he played the game, Astarion's voice actor Neil Newborn mentioned a secret area that only he and a few others know about. "There's even something I know about that you can't get to unless you do something that I don't think anyone's going to work out," he said.
Newborn didn't give us any hints on how we can access this content, but he says it's about 2 hours of gameplay, meaning it's not just a dialogue line no one has heard yet. Hopefully in time, someone will figure out what Newborn is talking about, or Larian could rip off the plaster and tell us where to find it.