Originally Posted by AngryGanker
According to my calculations if you're fighting a 1500 HP enemy with 10 armour. Assuming you have your level 3 ulti on them, having a stygian reduces their EHP by a further 160. About a 7% decrease in their EHP. It might seem small but I don't know.
unit with 0 armor or below has the same EHP as its HP
unit with negative armor takes additional damage by the formula:
Damage Increase = 1 - 0.94 ^ (-armor)
so if you use lvl 3 amplify damage (-16 armor) and stygian desolator (-7 armor) on a target with 1500 HP and 10 armor
target's EHP got reduced to 1500 and takes 1 - 0.94 ^ (--13) = 55.2635% additional damage from physical attack