The quest can only be accepted once "Kindred Judgment" has been completed.
They've holed up in an abandoned fort in the Rift. Killing their leaders should discourage an all out war with us."
In order to complete the quest, the Dragonborn must travel to Fort Dawnguard and simply kill the leaders who are indicated by objective markers. The leaders are:
At first, they will be unaware of the Dragonborn's presence, allowing for some stealth attacks, but once alerted, they immediately turn hostile due to the Dragonborn being a Vampire Lord.
This is despite Isran having met the Dragonborn earlier during "Dawnguard" and sending them to Dimhollow Crypt during "Awakening", indicating that Isran must have assumed the Dragonborn either fell to the vampires in the crypt, or became one of them when they never returned with news of what was going on there, forcing him to rebuild the order himself and prepare for possible war with the Volkihar Clan.
Once Isran and the others are dead, return to Castle Volkihar to report to Garan or Fura that the Dawnguard won't be a problem anymore to complete the quest.
|Destroying the Dawnguard – DLC1RV10|
|10||<Questgiver> has sent me to destroy the Dawnguard leaders at Dayspring Canyon.
|255||<Questgiver> sent me to Dayspring Canyon where I destroyed the Dawnguard leaders.