The Casivallan is a warlock who was created by the ancient druids with the intention to sweat the impending invasion of Rome. But the plan backfired. Casivallan sucked the live power of the surrounding and became therefore highly powerful and long living.
He calls for Harry and 6 virgins (Luna, Ginny, Hermione, Daphne, Eloise and Susanne) because Harry’s power burns extremely light but also short. While Harry spends two month with each girl in the past with lots of sex involved, Casivallan remodels Harry so that he can defeat Voldemort and live a life.
DrT writes:
The summer after OotP, Harry spends time with six of his attractive classmates as he goes on a quest through time for the one warlock who has the power to help him defeat Voldemort. Will Harry find love, or at least have a good time? Featuring Ginny, Daphne, Susan, Eloise, Hermione, and Luna in a very unserious tale.
The Story can be read at The Astronomy Tower.
There is also a PDF of the story.