At the end of Harry’s seventh year Voldemort is defeated; but at what costs. Virtually everybody but Harry and Luna are dead. Therefore Luna finishes a spell that her mother started 8 years earlier to send Harry back in time to give him a second chance. It works, although Luna dies but on the ‚bright‘ side the ghost of Hermione returns too. So Harry goes through all this again but with superior knowledge and all his magical power. Therefore all Horocrux‘ are destroyed at the beginning of Harry’s first year. In the summer after his third year he defeats Bella and Voldemort – who is her second – in an official duel. Afterwards he and Hermione, with whom he is magically bonded, disappear from Britain.
DrT writes:
The Last Battle has been fought, and Harry Potter has won. The price, however, has been high. Nearly every person Harry cared for is dead, maimed, or otherwise injured. The magical culture of Britain might never recover. Luna Lovegood, however, knows a secret which will give Harry a second chance, a chance to make a better future for himself and for most of the people he has cared about.
The story can be read online at FicWad.
There is also a PDF of the story.