Essence of Pain is reagent that is used to by Rogue to create poisons. Poisons that can be made with Essence of Pain are 
[Crippling Poison] Rank 1, Mind-numbing Poison Rank 1 and 2 and 
[Wound Poison] Rank 1 and 2.
Source
Essence of Pain is usually sold by vendor near Rogue trainer in capital cities. Usually price is 50 copper.
Essence of Pain as an Ingredient
Poisons
[Crippling Poison] (Rank 1)
Mind-numbing Poison (Rank 1 and 2)
[Wound Poison] (Rank 1 and 2)