Lantern of Revelation’s debuff is 8% at every rank. It should be instead 10% at green and 16% at mythic. I’ve also found some Lanterns acquired in the first modules that grant 10% a debuff that doesn’t increase with ranks. Vanguard’s Banner should decrease enemies’ damage resistance by 8%. This debuff is active only for less than a second, then it disappears and it grants instead 8% resistance ignored. This means that the debuff does nothing if the player’s armor penetration is capped. If a skill that deals damage over time is casted before or right after the Banner, the DoT keeps the correct debuff. As already reported, Pillar of Power and Bear your Sins have the same bug. Heart of the Black Dragon applies a debuff to enemies’ damage resistance, but it isn’t mentioned in the tooltip.
It was more or less between 7% and 30% during solo tests. I don't know how it behaves in parties, but I don't think it can reach crazy values like 100%.
That was how you told me it works, or how I understood how you told me it works and I trusted your explanation. But whatever, I am not even sure I care to defend myself anymore.
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I believe Sharp called it the "reverse prepatch Chromatic Storm": it DE scales with your team's buffs/debuffs.
So the more stuff your team stacks, the worse the debuff from Black Dragon becomes.