Instructions
Enter the Fade on your mage main character, either through Jowan's ritual or by asking the Circle Tower for help.
Work through the Fade, defeating any enemies you encounter, until you reach the desire demon.
Start a conversation with the demon. Avoid any dialogue options that involve threatening the demon or turning down the deal. Unless your character has the Master Persuasion skill, this includes the option marked with "Intimidate," as that will lead to a second, harder intimidation check that you can only pass if your character has that skill.
Continue through the conversation until the demon offers you a choice of rewards.
Choose the dialogue option corresponding to the reward you want. You can choose between unlocking the blood magic specialization, boosting the approval rating for one of your companions by 20 points, getting a talent point or seeing a kissing scene between your character and the demon.