Hi, since reductions like the one from the viper devotion or from items like open hand of mercy or empowered eternal band apply only to attacks with weapon%, the effectiveness of the reduction is also determined by the weapon% or it always applies in full?
Determined by % weapon dmg but cant exceed 100% of original value.
so only weapon attacks or high weapon% skills are useful in order to deliver this reductions.
I honestly don’t unterstand the difference in treatment between physical attacks and skills like Aether Ray in this regard…
Yes.
Well its triggering mechanic, some things are triggering some other things and some things dont 
I think that particular situation is more connected to game engine capabilities/limitations than a developers choice, tho I might be wrong about it.
Waaaaait a second, so if I use “Essence of Chton” augment on my Vitality/Chaos caster conjurer I can only apply it with Chaos Strike or default weapon attack??
Yup. Only through % Weapon Damage.
Anything with a % weapon damage component. Below 100%, the applied effect scales appropriately.
Anything that is alone (meaning it’s not applied through a skill) is applied through weapon damage. Stuff like flat damage on devotion nodes, items and active/passive buffs, same thing for resist reduction, OA/DA reduction and ADCTH.
God damn it, so it means that Essence of Chton is useless on a caster except when Chaos Strike is used (and Flame Torrent/Twin Fangs devotion procs that have weapon damage).
Why the hell there is ADTCH on Fang of Chton then?
Because Fang of Chthon is not meant only for casters. It’s actually a pretty awesome item for dual wield Chaos builds.
No reason you can’t do Chaos/Vitality melee with it.
i see, so many mechanics clear now, thanks, guys