Assuming that Thrassian Stranglers actually works and isn’t artificially difficult to keep the stacks up on, then in PvP I will be running a Stage 3 Vampire Magcro, who runs Thrassian Stranglers and stacks a good deal of health.

Why stack health?

The concept here would be that Thrassian Stranglers and the Vampire toggle and passive can provide an absolute boatload of spell damage. The issue would be the 40% extra damage taken. If facing sustained damage, I don’t think that would actually be a huge problem, since the extra spell damage will make your heals be so high that you could essentially heal through the same stuff you’d be able to heal through without Thrassian. So the bigger issue is burst damage—which you’d obviously be very susceptible to when using Thrassian. But I think you can stack >40% extra health pretty easily to mitigate this (whereas it’d actually be difficult to stack 40% *extra* damage resistance into a build). If you’ve got >40% more max health than a normal build, then you won’t actually be easier to burst down.

Why Stage 3 Vampire?

Beyond the spell damage from the Vampire toggle and passive, a big reason to go Vampire here is that Undeath actually *will* mitigate the extra damage from Thrassian when you are low health. And since you’re a high-health build, there actually will still be a lot of health to work with when that happens. With massive heals due to Thrassian and the extra damage mitigation essentially eliminating the extra damage taken, you’ll be very hard to kill if someone got you low. And it’d be a dangerous game to get you low and fail to kill you, since you could use the Vampire spammable to output extremely high damage. Similarly, Mist Form will more than mitigate the extra damage taken.

Why Necromancer?

There are several reasons that Necromancer fits this build the best. The biggest one is that Necros are hit the least by the 20% cost increase from being a Vampire. Blastbones, Skeletal Mage, and Mender are very cheap due to the Reusable Parts passive and Summoner’s Armor, so a 15% cost increase isn’t actually a big deal. Meanwhile, the Tethers and Bitter Harvest are free anyways. Obviously, there will still be some abilities where the 15% cost increase hurts, but if you’re using the Vampire spammable and stun and mist form, then the decrease in Vampire skill cost actually probably will leave your sustain in a really good place overall, as a ton of your abilities will be really cheap. Meanwhile, Necros are also good for this because the 10% damage mitigation from Mender will help a lot.

Potential Build Details

This is more of a general concept, and less tied to very specific gear sets (besides Thrassian), but you could do something like this with the gear:

5x Plague Doctor
5x Warrior-Poet (or Vampire Cloak, depending on whether you use Psijiic ult)
1x Thrassian Stranglers
1x Stonekeeper (or Domihaus or whatever other 1pc bonus you want)

And the skills might look like this:

Front Bar:

Arterial Burst
Stalking Blastbones
Hypnosis
Sated Fury
Inner Light OR Elemental Drain OR Degeneration (if not wanting to use spell power potions)
Swarming Scion

Back Bar:

Summoner’s Armor
Mortal Coil
Elusive Mist
Spirit Guardian
Healing Ward OR Resistant Flesh
Temporal Guard

I’m on console so I can’t use the PTS, but just fiddling around on UESP build editor, I believe this would result in huge damage to the point of over 8300 spell damage and over 30,000 magicka, in no-CP. This would result in things like a roughly 20k damage tooltip on Blastbones in no-CP. Meanwhile, your health would be over 35k, meaning that even not taking into account the damage mitigation sources you have, that’s the effective equivalent of over 25k health in no-CP, for the purposes of how easy you would be to burst down. Which is actually quite a lot in no-CP, particularly when you take into account the pretty high resistances (over 23k each in no-CP) and multiple sources of damage mitigation. And, in terms of healing, you could have things like an almost 20k Resistant Flesh tooltip, an almost 30k tooltip on Tether, and/or a Healing Ward that could potentially give you an over 12.5k damage shield in PvP. The sustain wouldn’t too be bad either, I don’t think. In the build I quickly put together, you’d be looking at a little over 1400 magicka recovery in no-CP, but that’s while wearing heavy armor (i.e. you’d have the Constitution passive), and a boatload of your abilities would either cost only around 1000-1400 magicka or be entirely free. Elemental Drain could be used if it was an issue. And stamina sustain would be pretty good too, since you’d effectively have almost 1000 stamina recovery in no-CP counting Mortal Coil, as well as the Constitution passive.

I think this would be really strong, as long as Thrassian isn’t glitchy. I suspect it wouldn’t work in BG’s, since you probably lose the stacks you have when you load into the BG, and it would be quite difficult to quickly get those stacks up. But in Cyrodiil and Imperial City, it would probably work—particularly with the NPCs you could farm. That said, if the ability falls off when you do things like enter/exit keeps or something, then it might not be viable.

EDIT: I made some changes in recognition of the point that this wouldn’t synergize well with the Stage 4 invisibility passive. Seems likely this should instead be Stage 3.