If you choose the Dawnguard faction, you'll miss out on the Rings of Blood Magic. They are the Ring of the Erudite, easily one the strongest ring for mages in the entire game, and the Ring of the Beast, which improves your health by 100 and your unharmed damage. This mod remedies that: The two rings have been distributed in the world to characters that will make good use of them. it's always been a pet-peeve of mine that the strongest items in Skyrim are sitting in chests taking dust, rather than being used. Now the Caller will wear the Ring of the Erudite, and the Ring of the Beast can be found (quite fitttingly) on Borkul the Beast.

- The Caller has a really interesting concept to her, but she's just a run of the mill conjurer, and doesn't drop any interesting loot. With the Ring of the Erudite, she'll be tougher, and drop a unique ring. You'll meet her during the college questline.
  
- Borkul the Beast is supposed to be this huge, imposing and tough orc, yet he goes down quite easily. The Ring of the Beast will make him more resilient and deal more damage. You'll encounter him during the quest No One Escapes Cidhna Mine.

If you wish to start a new game, and join the Volkihar, simply disable/disinstall the mod in order to prevent duplicated items. You'll keep the items in your original save file. 

If you have already defeated Borkul and/or The Caller, you won't be able to obtain the rings. If so, and you haven't completed the Dawnguard questline and want to nevertheless obtain the rings, I suggest you install only one of these mods:

Ring of the Erudite and Ring of the Beast for the Dawnguard path
Ring of the Beast v2 plus Rings of Blood Magic acquisition for the Dawnguard path

Or use console commands!

Update 1.1:
The mod is now flagged as an esl, so it won't count towards your esp limit. 

How to update:
Replace the esp in your data file or select the replace option with your mod manager.

Installation:
Use a mod manager or dump the esp in your data folder.

Compatibility:
Should be compatible with any mod that does not alter either Borkul or The Caller.

Roadmap:
If enough people are interested, I might add optional version of the esp which also makes Borkul and The Caller stronger. I had these changes in mind:
- Give Borkul the perk "Fists of Steel", and enchant his Iron Gauntlets to increase unharmed damage.
- Give better gear to The Caller. Her robes add 50% magicka regen, really? random necromancers have better gear than that!
- Buff the Ring of the Beast, and allow Werewolf form benefit from it. 

I might also consider creating a patch for The Caller - DX Necromancer Robes.