When you ask the hunter who this Maiden Fel is, he says she is the reason even monsters have nightmares.
Astarion is implying the Gur was acting indirectly on Cazador's orders, but he could be trying to misdirect the PC here. He is not willing to confess what exactly he did to the Gur in the past. Maybe he realizes at this point that the Gur could also be after him because of that past, and he doesn't trust the PC with this.
Try as I might, I could not re-create this conversation where
the Gur names this Maiden Fel. I tried first with my Tiefling Druid who was romancing Astarion and had High approval with him, but was well past the point of knowing he is a vampire - so the only option was for the whole team to fight the monster hunter, whose corpse naturally wouldn't speak to any of my characters afterwards. I then tried with my Drow who is at Neutral with Astarion, and does not yet know he is a vampire, so Astarion shanked the monster hunter and I did get to speak to the corpse afterwards - but the corpse wouldn't name any names.
How did you guys even get this bit of information to reveal itself in the game? :O
Seriously can't wait for more of the story now.