Talk to Fura Bloodmouth in Castle Volkihar. She is usually found either wandering about the castle, near the smithing station, or in one of the coffins. She tells you talk to Feran Sadri who has an assignment for you. You receive the objective:

Speak with Feran Sadri for an assignment.

Find Feran Sadri. He is often in the main dining hall, or in a coffin at the lower level of Castle Volkihar. If you cannot locate him, open your quest panel and go to the Misc quests tab, then set the objective to trackable. Ask Feran what you can do to help. Feran tells you he has discovered the whereabouts of a set of ancient Rings of Blood Magic. The belong among our kind, not gathering mold on some vagabond's treasure pile. They increase the abilities of your vampiric powers. Molag-Bal himself gave these rings to ancient powerful vampire lords. Harkon himself disbelieved in their existence, though perhaps he felt slighted they were not already in his possession and only feigns indifference.

Feran says you will be able to keep these rings, should you find them.

This starts the quest:

Rings of Blood Magic
Feran Sadri has sent me to retrieve the Rings of Blood Magic

Retrieve the Ring of the Beast from Orotheim
Retrieve the Ring of the Erudite from Crossroads Watchtower.

Note: The location of the rings can be in any dungeon across Skyrim. This will be selected at random. For example you could be sent to Hag Rock Redoubt, Volksygge, or Dead Crone Rock instead.

You can get the rings in any order. The rings will be either in a chest, or on a boss (Defeat, and loot). Travel to the first location and find the Ring of Beast:

Ring of the Beast
While wearing this ring, you will receive 100 extra health and your claw attacks will do an additional 20 damage.

Travel to the other location, and find the Ring of Erudite:

Ring of the Erudite
While wearing this ring, you will receive 100 extra Magicka as well as regenerate Magicka faster.

Note: Some of our readers have reported a bug in which the marker points to an empty location. This is most likely because you have killed the boss who was carrying the ring earlier in your adventures. At this moment, there is no known fix for this bug. You could reload a save from before you received this quest, and talk to Feran again to hopefully get a different set of locations. Otherwise, you will need to wait for a patch should this happen to you.

After retrieving both rings, return to Feran Sadri. Tell him you have collected all the Rings of Blood Magic. He trusts you will put them into good use.

This completes the quest, Rings of Blood Magic.