This quest requires you to have Spirit Vision ability, otherwise, you won't be able to complete it.
This quest is connected to Red Ink In The Ledger quest. If you follow the blood trails you not only find the cargo but also a shallow grave with Liam's corpse. When you search them, you can find Trader Limb. If an elf eats them you learn that this corpse belongs to someone called Liam who was murdered. Use Spirit Vision ability and you will be able to see Liam's ghost.
Speak with him and you learn that Garvan is the traitor. The ghost wants revenge. He asks you to murder Garvan and bring his head to the ghost.
Garvan can be found in The Black Bull. If you decide to kill him there your opinion in that place will go down drastically. This means that you must lure Garvan out from The Black Bull. Buy a Meaty Stew from Lorvik and poison it with any of the Void-Tainted Fish. You get a Tainted Stew that you need to give to Garvan. After that he will go to a more secluded place. Follow him and attack when you get a chance. Search his body and take his head.
Ending 1: You gave the head back to the ghost
Take the head back to Liam. He is very grateful that you have avenged him.
Reward: 6950 experience points + 2 guaranteed rewards + 1 from 4 that you can choose from depending on character's class
Ending 2: The head was eaten
The head can also be eaten by an elf. By doing that you get Trade Secrets ability. Liam isn't happy that you didn't fulfill his request.
Reward: 5575 experience points + Trade Secrets