In this PC Gamer article about the craziest DPS builds of all time, they describe an OP mDK Vampire PVP build from 2014. Although in the linked video it's clearly stated the mDK does little damage it still takes 30+ players to bring her down. Wasn't there for those days but Cyrodiil looked rock solid back then.
http://www.pcgamer.com/the-craziest-dps-builds-of-all-time/
https://www.youtube.com/watch?v=Uq2-Zl1PETM
From the article:
Ever wanted to be a raid boss in an MMORPG? You could in 2014, not long after the launch of Elder Scrolls Online, if you were a magicka-focused Dragonknight who'd become a vampire. You were both DPS and tank, able to take on dozens of players in PvP at once and kill most of them as well.
The root of the problem was the vampire tree's morphed "ultimate" ability, Devouring Swarm, which sent a swarm of bats down on everyone else in melee range while also healing you for everyone hit. But every class who became a vampire had access to that.
Dragonknights, though, could also use their Dark Talons skill to root all those players in melee range while roasting them with fire damage at the same time. A huge magicka pool made it even deadlier. Then a passive ability called Battle Roar factored in, allowing the Dragonknight to replenish health, magicka, and stamina based on the cost of casting Devouring Swarm. And it gets crazier. If you were wearing the Akaviri Dragonguard Set, you enjoyed a 15 percent deduction in ultimate ability costs, essentially allowing you to spam Devouring Swarm.
This was already hellish with regular Dragonknights, but players who earned the "Emperor" title in PvP might as well have been Daedric lords. Being the current emperor granted buffs like 200 percent ultimate and resource generation, leading to situations like the one in the video above.
The easy way to stop this nonsense was always just to stay out of range (although the DK's charge ability from the Sword and Shield line complicated that). Within a month, though, ZeniMax Online nerfed it to hell.