Talk to Garan Marethi (Castle Volkihar) to begin this quest:

Protecting the Bloodline
Garan Marethi, concerned that additional vampires in the area will cause problems for the Court, has sent me to Lost Knife Hideout where I am to destroy the Master Vampire before it turns its allies into vampires

Kill the Master Vampire at Lost Knife Hideout

Note: The location of the Master Vampire is Radiant, meaning it can be different any time. For example, instead of getting sent to the Lost Knife Hideout, you could get sent to Halted Stream Camp, Falkreath Watchtower, Embershard Mine, Broken Oar Grotto, or some other dungeon. Also, sometimes you will get this quest from Fura Bloodmouth instead of Gran Matethi.

The remaining of the quest is the same. Go to the dungeon, fight the native enemies there, and track down the Master Vampire who will usually be towards the end of the dungeon, if not in the very last room.

Lost Knife Cave is located in the Eastmarch Hold, near Fort Amol to the northeast of Ivarstead. However, as mentioned, the location you received could be different, so after setting the quest as active in your quest log, track it in the map and fast travel to the nearest waypoint. Enter the dungeon and kill the Master Vampire.

Once you are done, return to Garan Marethi back in Vampire HQ in Castle Volkihar. Tell Garan you have killed the Master Vampire at Lost Knife Hideout. As a reward you will get Potion of Blood (3) or some leveled enchanted item. Check with Garan and the other vampires whether they have additional Vampire Lord Radiant Quests for you.

This completes the sidequest, Protecting the Bloodline.