Vampire Faction -- Culling the Beast - The Elder Scrolls V: Skyrim - Dawnguard
Talking to Feran again causes him to direct you to speak with Fura Bloodmouth, who it seems has something for us. In fact if you try to help any of the other merchants in the Castle they say the same thing -- so we should speak with Fura Bloodmouth eh?
You will very likely find her near the Smithy practicing her weapons skills -- and when you talk to her she begins by giving a long discourse on, well, her. She has some chat options you can explore, and she can train you in Two-Handed if you still need it... When you ask her about helping out she tells you that there is a beast that needs killing -- specifically a Master Vampire at some random location -- and this flags the quest CULLING THE BEAST.
Culling the Beast and Assassinating a Master Vampire
Since this quest involves killing a Master Vampire and we will not be interacting with the Herd we don't have to concern ourselves with what stage we are presently at, though you will want to make sure your kit is in order. When you are ready to go depart the Castle and fast travel to the location that was specified (or as close as you can get). The location varies as this is a repeatable quest that can actually be given by other members of the collective than Fura -- just so you know.
When you arrive you should know what to do -- kill! Weather you do that with your sword or with your vampire powers is your call -- I find it a lot faster to use the sword myself. Depending upon which location you get sent to you may face dogs, wolves, vampires of different types, and thrall, and then the Master Vampire you came to kill. When you are done head back and report in!
Your reward is three bottles of Potions of Blood and the knowledge that you have once again performed a service for the collective. Well done!