Romulo's Poison Vial

Romulo's Poison Vial is an epic trinket.

Source
This item drops from Julianne in Karazhan.

Theorycraft
According to simple tests, this item has no internal cooldown and just a probability of about 3% per attack. (Patch 3.0.2)

Trivia
From Romeo and Juliet: When Romeo thought Juliet had died, he ended his life by drinking a vial of poison.