Fort Dawnguard, on top of a barrel in the torture room.
When read, it starts the quest 'Ancient Power' to retrieve the five Ancient Vampire artifacts across Skyrim.
|The subject has revealed under torture the whereabouts of five relics the vampires believe can imbue their kind with additional abilities, making them particularly destructive to mortals. These relics are said to be the remains of ancient vampires, with rituals intended to utilize ancient blood to enhance vampiric abilities planned.
It is well known that the vampiric bloodlines convey a vast variety of supernatural abilities. Similarly, vampires have always believed that the most ancient of those bloodlines convey more power than younger bloodlines. Our records show numerous wars between vampire clans that ended with the victors consuming, quite literally, the defeated. Some were even waged for this express purpose.
It is difficult to determine whether the victorious clans truly grew stronger from the cannibalism, however if there is any chance these remains do contain the capacity for the enhancement of vampiric abilities, it is in the best interest of mankind that they be obtained by any means possible, and subsequently destroyed. Stendarr willing, placing them into the fire at His Beacon will be sufficient to destroy them. Otherwise, we're going to need to dig a really deep hole.
Per the subject, the remains and their whereabouts are as follows: