You watch Vanessa writhing on the floor and notice she's starting to change. Her skin is turning red and soon horns poke through her hair, much like your own. She rolls over on her front as her shoulder blades seem to swell until black wings burst from them. At the same time the skin above her perfect ass stretches into a long sinuous tail. You marvel as your girlfriend becomes a succubus in front of your eyes. You're fascinated and aroused by the transformation - frustratingly aroused by this point as Vanessa had pleasured you before her transformation without bringing you to climax either. You focus, trying to direct your tail to pleasure yourself like you had pleasured your girlfriend with it, but the long appendage bends through your legs again without having enough flexibility for the desired purpose.
You look down to see Vanessa is finished - both her transformation and orgasms. She slowly stands and looks at herself in her nearby mirror. She runs her hands over her new succubus form then turns to look at you.
"You did this to me, didn't you?"
"Yes."
"However can I repay you?"
You have but one great unfulfilled need.
"Finish me."
"With pleasure" Vanessa says, licking her lips with her new tongue.
She flings you on the bed then leaps on after you. Before you can say another word, her face is buried in your crotch and her supernatural tongue is inside you. Now it's your turn to thrash around, clutching the bed as your body shakes. She pushes you until you can orgasm no more, even your supernatural stamina spent. Realizing her work is finished she crawls up next to you. The two succubi fall asleep in each others arms and wings.