Your damage is increased by 4.0% for each of Shadow Word: Pain, Vampiric Touch and Devouring Plague on the target. During Voidform, all targets receive the maximum effect.
At level 60, the ratio is 70 mastery rating per 1% mastery.
Diminishing returns from additional mastery rating start at 19% mastery (rather than the standard 30% for other masteries).
Shadow Weaving is unable to increase the damage dealt by [Shadowfiend] and [Mindbender] directly. However, the caster still receives the "benefit" from this mastery in a unique way:
When either pet damages a target, additional shadow damage is passively dealt by the caster, equal to the bonus damage provided by this mastery.
This damage is listed in the combat log as Shadow Weaving.
Trivia[]
This is the fifth mastery shadow priests have received since masteries were introduced in Cataclysm. The former four masteries in order:
[Shadowfiend] and 
[Mindbender] directly. However, the caster still receives the "benefit" from this mastery in a unique way:
Patch 9.0.1 (2020-10-13): Added. 
