Apparently that's not quite necessary, because Cazador can replace Astarion in the ritual and Cazador didn't bring Astarion any spawn. Cazador did make all the spawn, of course - I guess that's enough?
The door unlocking as Astarion approaches is sort of a sign that Cazador knows he's there. Maybe. But the ritual is already underway, isn't it? Or wasn't it? I dunno.
