Disclaimer: I am not J. K. Rowling. I do not own Harry Potter.
Note: The following assumes an alternate universe in which Harry found about the fragment of Voldemort's soul that had been in him ever since Hallowe'en 1981 during the autumn term of his sixth year. The majority of this piece takes the form of a letter written by Harry Potter to Lily Potter.
Dear Mum,
If all has gone to plan you will be reading this letter and be alive once more. You once offered your life in trade for mine, and now I give back the gift.
There wasn't any alternative that I could see. Professor Snape abducted me from Hogwarts, to try and save me or something. When You-Know-Who attacked us that night on Hallowe'en, You-Know-Who apparently lodged a fragment of his soul in a scar on my forehead. As long as I'm alive he can't be killed, or not permanently. Professor Dumbledore's terminally ill or something and wanted Professor Snape to make sure if the headmaster died first that I would die 'at the right time', but Professor Snape for some reason kidnapped me instead, because he refused to see me hurt. I can't figure him out. He spent the past five and a bit years being a git to me in potions classes, and I would have thought he'd jump at the chance to see me die, but it turns out not. It's not even as if he did it for You-Know-Who, either – he's on the run from not just Professor Dumbledore but from him, too. I would have thought if he didn't want me dead, then handing me over to You-Know-Who so I could be put permanently to sleep or something and ensure You-Know-Who's immortality would appeal to Professor Snape.
Anyway, Professor Snape explained why he'd abducted me – well not his reasons, but just the bit about the soul fragment and that he didn't want me dead because of the headmaster or You-Know-Who – but despite his best attempts to keep me alive and where he could keep an eye on me, I managed to get away from him, and went to ground in Grimmauld Place. I inherited it from Uncle Sirius. By the time the Ministry finally figured out Uncle Sirius hadn't been your secret keeper and betrayed us, Uncle Sirius had been to Azkaban for over a decade, escaped, and spent his last couple of years living as a fugitive on the run from 'justice' – before being killed by his cousin Bellatrix. I'm going to try and leave Grimmauld Place to a great friend and mate of mine, Ron Weasley. He was in my year at Hogwarts and we had a lot of fun together, even if at times he was a bit jealous of me for being famous and having people try to kill me. His sister Ginny (full name Ginevra Molly Weasley) owes me a favour for saving her from a basilisk and I've got her to help me with this. There's a spell I found in the Black family library at Grimmauld Place, you see: bring back someone killed before their time by a willing sacrifice of an immediate blood-relative. Apparently parents used to use it for their children, but I'm doing it the other way round. Sure, there are other spells powered by sacrifice for bringing vengeance on enemies and the like, but they all look like the sort of thing You-Know-Who would use, and I don't want to be like him. If I'm going to die, I want to do something with it which is the opposite of destroying things, and the world could use you, mum, from what everyone I've ever heard talk about you says.
This is a lot harder to write than I thought, and Ginny's calling for me now. It's time-sensitive – the spell – and we can only do it at the turning point of the year, on the winter solstice, where the days begin to lighten again. There's no more time to write. You can talk to her about me if this works.
Your son,
Harry James Potter.
PS
This magic is a one-way deal. Someone who gives up their life using it can't be brought back with it. Don't even *think* about trying that.
Slowly, the redheaded woman sitting in the armchair of the study put the letter down, silent tears running down her face.
Author Notes:
This is alternate universe. As far as I can make out, in canon Albus Dumbledore doesn't give Professor Snape his 'Harry must die by Voldemort' speech where he reveals the soul fragment connection between the two until some point after Christmas. For whatever reason, in this universe Albus Dumbledore decided to tell Professor Snape during the autumn term with the result that Professor Snape almost immediately abducted Harry from Hogwarts (although making the mistake of explaining to Harry just why he had done so). Since Harry hadn't reached the point in his 'lessons' with the headmaster by then where the headmaster had actually informed Harry that Voldemort has multiple horcruxes, as far as Harry knew, he was the most significant (if not only) thing keeping Voldemort around - and between depression over Sirius' death, his life to date, and that the headmaster wanted him dead, he decided that he had to kill himself. (Canon Harry is fast enough to commit himself to suicide-by-Voldemort in 1998, the moment that he knows that it's necessary.) Inspired (to some extent) by his mother's sacrifice, this Harry did check the library at Grimmauld place for any magic he could usefully incorporate his death into. Although I've taken the liberty of imagining the piece of magic which he does find and use, it would seem to me that a piece of magic which would allow a (willing) blood-relative to trade their own life to restore a close family member is something which might be in keeping with the world in which Harry Potter exists.