Negative Armour Clarification

Though it seems to be bugged at the moment anyway (https://www.pathofexile.com/forum/view-thread/3615635), it would be nice to have some more information about how negative armour (Warbringer node) is supposed to work.

Assuming that the damage reduction (DR) formula is similar to PoE 1, simply using negative values for the monster's armour would lead to unintended side effects.

DR = Armour / (Armour + 5*DMG)

If the hit's damage is 100 and the monster has -499 armour, then the DR is -499 (or 49900% more damage dealt). If the damage is less than a fifth of the absolute negative armour, then the DR would be above 1, i.e. healing the monster, unless the max 90% cap is still active.



What would make more sense is if the DR gets flipped once armour is negative, i.e. instead of a "less physical damage taken" modifier it now works as a "more physical damage taken" modifier on the monster. This would also have the interesting side effect of boosting smaller hits more than large ones (as the DR inversely scales with the DMG magnitude), which could help builds using a one handed weapon and shield, instead of a two handed one.