Talk to either Aela , Farkas , or Vilkas to get this quest, whom I will be calling " The Circle Members " from now on. Apparently a group of vampires has been causing trouble in Eastmarch , so we need to clear out Mara's Eye Pond .
Note 1: These are Vampires we're talking about, so if you don't wish to contract their disease bring along a cure disease potion.
Note 2: Due to the radiant quest system, this quest can also pit you against bandits or other types of enemies at other locations. Just FYI.
Mara's Eye Pond is just east of Gallows Rock, or just southwest of the big town of Windhelm . It's in the middle of a lake, accessed by a trapdoor. Smart! Head inside once you find it. There's one vampire in here and one fledgling vampire.
They aren't too hard to best, although they will try to raise a servant and sap your health. Take them out and explore the place to find all sorts of jewels. There are two chests to pick here too, as well as some cages down by the water.
Once they are dead, head back to whatever member gave you this quest and turn it in. You will be rewarded 300 Gold upon completing the quest.