Design:
Implemented random encounters where they can be found killing and filling soul gems with souls of necromancers, vampires.
--Some encounters at lower levels where they only stare and remain docile unless provoked.
Recurring event where they will attempt assassinating the Dragonborn, both on the road and in his/her sleep.
Recurring event where Dragonborn's allies, friends, spouses end up soultrapped and dead.
Quest to root out these things and stop them, where one can be made a follower.
Supernaturally powerful and universally capable of any manner of combat.
Absolutely ruthless, relentless, and kamikaze fighting style.
Can be stealth-killed by pickpocketing a special soul gem (like briarhearts).
--Instead of thugs showing up to beat up the big bad Dragonborn for stealing (weird, right?), a group of the creatures will appear instead.
Resists poison, enchants, magic; can only be killed with a physical (not conjured) weapon.

--Exception: Race-specific weakness to custom enchantments and spells that can pacify them.
Immune to paralyze and soultrap.
Nerfed with medium-low health, low defense, easily ragdolled and knocked back large distances with powerful attacks/bashes and unrelenting force shouts.

What they have and can do:
Summon/conjure.
Raise dead.
Use powerful magic.
Use weapons powerfully (leveled weapons that they can use better than you can).
Dual wield.

Move and attack 1.5x to 2x faster than you.
Attack in groups.

Should they have staves? Besides being limited, hard to balance and kinda lame, it just doesn't seem like something they'd have.
Debating whether or not to have them able to ride Arvak- or Shadowmere-like mounts. Or maybe just regular old horses. Can NPCs do mounted combat effectively, or at all? We'll see.

What they don't have and cannot do:
Shout.
Heal.
Use defensive spells.
Very little armor/defense.
Limited effectiveness in blocking.

Physical design:

Based off four human races.

4.5 to 5 feet tall humanoids, supernaturally fast.

As white as can be with solid black features (hair, makeup).

Semi-vampiric and/or undead traits.
Glowing eyes.
Exclusively female (due to backstory).
Dressed in the various monk/warlock/necromancer robes and boots, and something similar to the mystic tuning gloves.


Backstory:
Prospective Daughters of Coldharbour that didn't make it. The first pureblooded vampire was thought to have died too. Molag Bal drops that blood on them regardless of whether or not they survive. So the souls end up powerful, even if the body is pulverized, broken, smashed and torn. Soul ends up escaping Molag Bal's plane of Oblivion and end up in the Soul Cairn. Just as the Ideal Masters were interested in Valerica's soul, they take an interest in these souls as well and utilize them as corporeal agents in Tamriel, collecting the debts owed by necromancers as well as interesting specimens; the soul of the Dragonborn, for instance, or Serana post-cure sidestory.

Quest story:
No kicker. Activated from the start or upon installation. At low levels you see them creeping about and minding their own business. At level 50 or after the Dawnguard quest is completed, they begin pursuing the Dragonborn and a quest opens up to stop them. Keeping the rest to myself.

Requirements:
Obviously Dawnguard. If at all possible I'll try to avoid requiring SKSE or compatibility patches with the Unofficial Skyrim patches. I use these things and love them but not everyone does.