Speak to Takata Steelblade in Desolace.
This creature is far more dangerous than I had anticipated. No doubt Takata questions my abilities to take on such a creature, and it would insult his honor if he were not informed about what just happened.
Speak to the swordsman, tell him you are willing to slay the demon if he would allow it, but do not press too hard. It is better if he thinks it his own idea.