When you reach Redcliffe, you will be given the option of entering the boy or killing him and confronting the demon inside of the boy. Do not kill the boy. You will not gain much from his death and your allies will disapprove. Instead, choose the option to enter the boy and let the Blood Mage do his ritual. Once you are inside, talk to the father. Afterward, you must talk to the child "Possessed" and kill the demon inside him multiple times. Once you reach a certain part where you talk directly to the actual demon, try to converse with her. Conversing with her will get you more than simply killing her. She will request that, in exchange for her life and the permission to enter the boy's body again in the near future, she will offer you gifts. One is a new specialization, while the other is a certain rare item, armor, or weapon. The last option is pleasure, which should be avoided as it does not give you anything in exchange, not even an intimate scene. Select either the item or the specialization. However, if you truly do not want to obtain anything useful, kill her.