Location: Northeast of Cheydinhal, near the Temple of the Ancestor Moths (link goes to the Daedric Shrines map at UESP)
Minimum Level: 2
Offering: Glow Dust (get it from Will-o-the-Wisps or some alchemists) - must be done at dawn/dusk
Reward: Azura's Star (a reusable soul gem)
Objective: kill five vampires at the "Gutted Mine", southwest of the shrine.
There's a trip wire trap just inside the door. Be careful. The vampires are spread out, so it should be mostly one-on-one and a "Detect Life" spell would be very handy about now. Also, don't waste a "Cure Disease" potion or spell until the last one is dead. You probably will contract Porphyric Hemophilia, so if you want to take a walk on the dark side, this is definitely one of the places to do it.
Don't forget that you can sell Vampire Dust for 250 gold each if you have completed the "Order of the Virtuous Blood" quest.