Baldur's Gate 3 has a ridiculous amount of content, but not everything that was planned for the companions made it into the final game. Whether it was cut for time constraints or because the writers just took a character in a different direction, there are a bunch of unused conversations and scenarios that are still in the files, but typically inaccessible.
One such scene involves Astarion, and was only accessible during early access. Back then, as long as you were not playing as Astarion, you would get an event during a long rest in which the vampire wakes up from a nightmare. After this, you can talk to him about the nightmare, giving you one of your first opportunities to learn about Cazador.
This scene has been restored by Nexus Mods user HyperspaceTowel, and you can download it for yourself if you're playing on PC. I'm not seeing it on mod.io, so it seems like it isn't available on console just yet.
Once you've installed the mod, you can get the conversation to appear during a long rest. It will play if you're in the camp from the first area of the game, and as long as you have had...
