Princess Serena needed to humiliate this slave to teach him a lesson that no one had taught him. He misbehaved but she felt that it had not gotten to the point where she had to use pain. That is why she had to ensure that he learned his lesson in a humiliating but non-painful way. The mistress gave him blowjob training and he had to use a dildo to prove to her that he had learned.
Princess Serena does not like this sissy and she knew the best way to torture him and scare him not to mess with her or approach her or be seen anywhere near her was to do something he did not expect. And so she teased him a little and when he was turned on, she punished him for being turned on and she threatened to ass fuck him with a strapon.