Dame Mahou Shoujo no Otoshikata

A magical girl named Momone goes undercover at a massage parlor suspected of illegal operations. However, the staff member giving her a massage turns out to be a succubus who was also a former magical girl. The succubus uses her powers and tentacles to pleasure Momone, eventually transforming her into a succubus as well. The two then go to a hotel to test out Momone's new succubus powers. The succubus staff member continues to torment Momone, who is now completely brainwashed and enslaved. The staff member uses a magic lock on Momone's penis, ensuring she can only climax inside the succubus's pussy. Momone is then shown being milked by a machine while the succubus watches, declaring that she will return the favor for all the torment Momone put her through.