Destroying the Dawnguard

Walkthrough
After Kindred Judgement and a few other Radiant quests completed for the vampires at Castle Volkihar this quest will come up.

Kill the Dawnguard Leaders
Either Fura Bloodmouth or Garan Marethi will mention that they would rather avoid an all-out war with the Dawnguard, therefore it would be very much more efficient to take out all their leaders in one swoop.

Travel to Fort Dawnguard and kill all the members of the Dawnguard there, all eight of them.

Return to 
Returning to Fura or Garan will yield a pitiful reward, considering the significance of this mission.

Das Ende der Dämmerwacht