Risen

Chapter 1 (Harbor Town)

 Find Sonya's brothel in Harbour Town, to your right coming down towards the harbour from the Gutter district. Enter the bathroom to the left and talk to Gwen in the nearest tub. She will ask you to get rid of Erikson, who is sitting in the foyer. Go talk to him and provoke a fight. Remember to clean out his inventory once you beat him, but don't kill him. After Erikson gets up, talk to him again and tell him to leave and never come back. Go back to Gwen and talk to her to unlock this achievement.

Note that Eriksen is connected to another questline too, and have both a key on him and information you will need. To start this quest before beating up Erikson, talk to Patty in the tavern opposite the brothel, then Vasili who can be found in or around the tavern.