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After you’ve figured out that Milton is hiding in the Greenhouse from the last quest you’ll go there automatically with Anna Henrietta. Unfortunately you’re too late and the beast will have already him.
Chase the beast to an empty house and after a short conversation a fight will begin. You can loose the fight although if you do you won’t get any experience points.
This vampire is fairly tough and will teleport around to try to slash at you from behind. Keep dodging and have Quen active when possible. Towards the end of the fight Regis will appear and stop the fight.
Speak to Regis to find out the beast is a vampire named Dattlaff. You won’t have too long to talk before the Knights appear and Regis will have to flee. After you converse with the Knights the quest will end.