Deadly Delights

When you finish talking to the guard, head over to the docks and talk to the medic. Then head over to the indicated area and use your Witcher Senses to locate hoofprints. Follow them to Crippled Kate's [the brothel]. The door will be locked, but you can circle around the building to enter.

When you get inside, speak to the madame of the brothel. It may be helpful to have Delusion on hand to make her give up some information. Head to the back of the brothel and use your Witcher Senses to locate a bottle of perfume. Track the scent to a strange acting man. Head inside the house [if you don't have the key, you have to enter using the neighboring houses].

You have two options when you confront the succubus: you can fight her, or you can convince her to leave town. Whatever the case, head back to the guard when you're done to get your reward.