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@JonnytheKing remember that they changed default damage mitigation to be a flat number in the last patch too that might be the difference you are seeing.
Easiest test would be with/without the 5 set in effect on a player with no gear or passives then just progressively add damage buffs to yourself / debuffs to the enemy. Shouldn't take more than 30m to check it (likely quicker than waiting for a reply which could also be wrong)
@Solar_Breeze
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TLDR: Seems like the negative %damage modifier of Lamp Knight is simply additive with the total positive %dmg modifiers. Ex.: -46% + 16% = -30% dmg done. This is just an assumption as there are different damage buckets see Sparkrips document: spreadsheet
I did some testing on a dummy, so no battlespirit sadly. I might post later when a buddy is available.
But cba, the set is just badly designed. "Do damage but don't do damage". It should be enemies have zero armor but so do you. This way you can build around it, no need for armor abilities which frees up barspace, plenty of %mitigation options like esoteric, buffer of the swift, brp dw, etc.
As for the testing, the results are without cp, no skill passives, nothing else slotted but a spammable.
The result in yellow: basic 3m dummy with 9100 armor which is 18.2% mitigation (because dummies are lvl 50, not 66)
The orange result: same situation as yellow, but with a %dmg modifier like malacath.
The red result: outcome of the difference between yellow and orange for conclusion.
I spent 30 hours in Cyrodiil/IC with Lamp Art and finally decided to shelve it.
Best case scenario you're fighting an armor stacking player and do more damage than normal.
Worst case is your enemy has many sources of damage mitigation and not much armor to begin with. The damage from wearing Lamp Art goes way into the negative at that point. You're better off not wearing Lamp Art at all and just have a missing 5pc set in your build. No seriously, you're better off naked than wearing Lamp Art against low armor targets with high damage mitigation (duelist rebuff, ironclad, pain's refuge, vamp 3, etc)