Talk to Gunmar to get this quest. This is one of the Dawnguard Radiant Sidequests available to you when you choose the Dawnguard path. If Gunmar does not have this quest yet, do other side and main quests for the Dawnguard and try again.

Preemptive Strike
Gunmar has sent me to Cragwallow Slope where I am to destroy the Master Vampire before it turns its allies into vampires

Kill the Master Vampire at Cragwallow Slope.

Note: The location you are sent to kill the Master Vampire is random. For example, other than Cragwallow Slope, it could also be Gallows Rock, Orotheim, Falkreath Watchtower, and other locations. But other than the location, the quest objectives should be similar, just requiring you to enter a dungeon and kill a Master Vampire.

Open your map and locate the destination, which is a dungeon. Cragwallow Slope is located to the South East of Windhelm, however as mentioned the destination for this quest is random so look for the waypoint on the map.

You will have to fight your way to the end of the dungeon to find the Master Vampire. Enemies in Cragwallow Slope include different types of mages such as Conjurers, as well as summoned Atronachs. The Master Vampire will usually not be alone and have some "friends" to help him. Kill the Master Vampire.

Once you have defeated the Master Vampire, you need to return to Gunmar. Exit the dungeon and head back to Fort Dawnguard. Talk to Gunmar to complete the quest. Tell him you have destroyed the Master Vampire of Cragwallow Slope.

This concludes the quest, "Preemptive Strike".