The challenge is that Harry Ron, Hermione Neville Luna and Ginny are summoned by The Olympians and are told that Voldemort's reign is opening the walls of Tartarus and may unleash the Titans due to the negative energy and feeling Voldemort is spreading across the Wizarding World and Great Britain. They then train them not telling them they were teaching them to take over the Olympians place due to lack of prayer and belief of their existence the Olympians are dying. Here are the conditions:
Harry takes Zeus Place and Becomes King Of The Olympians and God Of The Sky and Lightning
Neville Takes Poseidon's Place and becomes The New God Of The Seas
Ron becomes The New God Of The Underworld and Dead
Hermione takes Hera's and Athena's Pace as Goddess Of Marriage and Wisdom
Luna becomes the New Goddess of The Moon and The Hunt
Ginny takes Hestia's Place as The Goddess of The Hearth
HarryXHermione
Harry and Hermione know who the Gods are.
What else you do is up to you.