Okay, we was able to test it more precisely with @Grumble_and_Grunt , and here are our conclusion :

- first point, and most important one : the damage buildup is totally broken. When you use only backslash (BS), the final hit is 14-18% of the stored damage. But when you use an another hit, each additional hit count for more : the second hit count for 2-3x more damage, the third for 4-6x more, the fourth one for 8-16x more, and so on. So, with 5-6 hits, you can easily reach the damage cap, even with light attack or tiny dots. Even if the target block, three or four people who only do light attack would easily reach the damage cap. That's the main problem of the skill.

- block mitigation is buggued with BS. we tried to use BS then lights attacks against a blocking target, both the light attack and the final hit has been reduced but the % reduction is not the same. It seem that these light attacks were counting for more of their actual damage for the buildup, but that can be a consequence of the buggued formula. Since we can't determine the exact buildup formula, we can't know precisely.

- resistance mitigation and buff system mitigation (from circle of protection in our test) work well, but one interesting thing is that the mitigation is calculated only when the final hit is released. If you loose all your mitigation just before the release, you'll take full damage as if none of them were mitigated. That's mean no double mitigation.
- Cp mitigation work fine too, it reduce the buildup but not the final damage (so no double mitigation too).

- mist form has his own way to work, and can reduce the stored damage, the final hit or even both, making it the ultimate counter for this skill (93,75% reduction, on a 10k hit it's enormous).

- finally, we can confirm that if you re-use BS before it end, you keep the stored damage for the new BS, allowing you to store the maximum amount of damage for whenever you want.


So, I think the core problem is how the skill take the cumulated buildup into account. We can determine that :
- for the buildup, the game count raw unmitigated damage (without mitigation, battle spirit and so on), but for whatever reason count each consecutive hit as a higher value than intended. Mist form reduction is applied before this part, so it's reduction is applied (unlike all other).
- for the release, actual mitigation (battle spirit, resistances and buffs, at least) are taken in account and applied to the raw damage stored.
Note that the mitigation given by armor/resistance work the same for pnj (tested with pierce armor, BS applied during the armor/resistance debuff, release applied after, no reduction).

What ZOS can do : look at the buildup formula. I think the mitigation part should be counted at this moment, and there is definitively a problem with consecutive hits here.


Thanks again to @Grumble_and_Grunt and to @Valves for both the initial testings and these tests, and to @ZOS_GinaBruno for looking at it

Just tested, each consecutive sweep hits apply the bug. So with BS + two sweeps, I reach the maximum damage.

And block mitigate during the buildup and not at the final burst, so this part is good.