A New Order

Serving as the fourth quest in the main storyline, "A New Order" begins with a vampire attack on the stronghold. The Dragonborn recruits two assistants to help with fending off the foes.

Walkthrough
Upon returning to Fort Dawnguard, several vampires attack the stronghold. After the battle, Isran asks the Dragonborn to recruit a Nord named Gunmar and a Breton named Sorine Jurard to help the cause.

Recruit Gunmar
Gunmar can be found at Pinepeak Cavern or another radiant location. He says he's been hunting a bear for two weeks and he first wants to kill the animal to stop it from terrorizing people. After helping him kill the bear, he'll head to Fort Dawnguard to see why Isran needs his help.

Recruit Sorine Jurard
Sorine can be found west of Druadach Redoubt and south of Darkfall Cave near a Dwemer ruin. She initially complains that she has lost her satchel of Dwemer Gyros and requires one before she agrees to leave.

Her large satchel containing seven Dwemer Gyros can be found on the river bank nearby, about three paces south towards the river near tree rooted in the rock. Alternatively, she can be persuaded to immediately go to Fort Dawnguard.

Return
After arriving at Fort Dawnguard, Isran tests both newcomers for vamprism shining a beam of sunlight into the center of the entrance hall. If the Dragonborn is a vampire, he will won't grant membership until the vampirism has been cured and will suggest that Falion in Morthal is visited. This begins the quest Rising at Dawn.

He tells Gunmar and Sorine about the current situation after clearing them. The quest completes and the next quest Prophet, begins.

Bugs

 * After killing the bear in Pine Peak Cavern, the quest never completes nor will Gunmar talk to the Dragonborn.
 * Isran and all of the other Dawnguard members will become hostile and attack. Solution: Exit to Skyrim, wait 24 hours then return to Dayspring Canyon and they should not be hostile.
 * Sorine wont accept the gyros from her satchel or from the player.
 * In Cronvangr Cave, the bear is nowhere to be found and Gunmar disappears.