A Feast for Crows
Side quest Novigrad suggested level 20
From the journal
Geralt spent some time rutting around in the dank alleyways of Novigrad and at one point came upon the body of a mysterious man. A cursory inspection revealed that this unfortunate soul had been struck several times with a sharp object. Though he seemed to have managed to flee his attacker and hide in this alley, he had bled to death shortly thereafter.
Geralt realized he could follow the trail of blood and find out who had done this to him - and why.
It turned out bandits had murdered the man. The witcher could not restore him life, but he could at least punish the guilty party. Which is what he did.
The bandits were guilty of murder, but Geralt realized even the harshest of punishments would not bring their victim back to life. Thus when they proposed a trade - the key he found on the body in exchange for gold - Geralt readily agreed and went on his way.