Quest given by Karthok the Ruthless
Search the Inevitable City for prisoner carts. When found interact with the lock to release the prisoner so they can defend themselves before you kill them for their head.
When you have the Severed Head return to Karthok the Ruthless and then interact with the Boiling Vat to place the head inside.
Prisoner carts are found throughout the Inevitable City. Upon opening the cart a lvl 6 prisoner will instantly attack.