Just so everyone is clear here: there are Briarhearts, and then there are BRIARHEARTS!!!1!1!!1
The OP is clearly referring to the latter. These are level 51 Briarhearts that dual-wield and are an absolute force of nature. From http://www.gamefaqs.com/boards/615805-/61362081
SO FAR the most powerful enemy I could find is a FORSWORN BRIARHEART (melee version, baseID: 44313). He can kill me at level 41 in a single powerattack with his dual wields. So I killed him and thralled him (with the "more powerful necromancy" mod so I can raise him). I spawned Orchendor, he died in 2 hits. Malkoran: 3 hits; his shade: 2 hits. I spawned a boss Falmer Shadowmaster, he also died in 2 hits. Elder dragon: dead in 3 power attack flurries. Dwarven Centurion master: cut down like butter by the briarheart. Otar the Mad (named dragon priest) dead in TWO HITS. Volikhar vampire master died in 2 hits as did a werewolf vargr. He even killed a clone of himself in two hits (the clone didnt fight back in time). This guy is like Kratos or Hercules or something.
He seems to have like 700hp and seems to do like 300 damage per swing with his damn forsworn swords. Of course this is just an estimate, but it takes me a while of hitting him with powerattacks to bring him down myself, even though he cannot block. Having him as my thrall breaks the game, all I have to do is watch him slaughter everything.
Anyone come across a more powerful enemy in Skyrim? I havent seen him fight an ancient dragon but I'm pretty sure if it landed next to him he could take it out by himself.
ETA: This is the one that killed me in 2 power attacks when I had 350 health and about 75% damage mitigation (adept difficulty). There is pretty clearly some hidden damage modifier that allows him to wreck stuff so easily, because a lot of enemies have 100 one-handed skill and they can't do what he does.