Necrosis

Dealing damage with Vampirism, Life Tap, Howl of Death, Wandering Fever increases the number of stacks of Neurotoxin by 6.
Blood Feast increases the number of stacks of Neurotoxin (if it's present) on all targets within range by 6.
If Advanced Genetics is unlocked, Ritual of Plague and Howl of Death will increase the number of stacks of Neurotoxin (if it's present) on all targets within 8 yards by 6.