Viper

is a tier one Chaos Constellation, located in the center of the devotion window.

Stats
Note that the 20% Reduced target's Elemental Resistances for 3 Seconds is tied to Weapon Damage, and is best used with weapon attacks, or spells with a high % Weapon Damage component.

A spell without % Weapon Damage will not apply the Resist debuff to enemies at all, and where it does have % Weapon Damage, the value will act as a multiplier for the Reduced Resistances; for example Callidor's Tempest, with 12% Weapon Damage, will only lower enemy resistances by 12% of 20% ( i.e. 2.4%). However, even if Shadow Strike (Skill) is dealing 300% Weapon Damage, it will only reduce enemy resistances by 20%.