Disclaimer: I in no way own any part of Harry Potter. I am making no profit off of this.
The spirit is not pleased.
The fact that all things eventually fade is a defining characteristic of the afterlife. Although space and time suddenly become infinite for the newly dead, and in some cases malleable, there is no corresponding increase in human cognition or memory capacity to match.
Human? Was that what he had been once? The memory flickers briefly, tantalizingly just out of reach, before slipping away.
Just like for the living, old memories will eventually fade. Spirits who remain long enough eventually forget every part of their life and became true natives of the heavens. They still possess the same basic personality characteristics – their experiences have directed their mental schema of the world to grow and work in certain ways, and that can not be changed except by new experiences. Instead, it functions more as a mellowing. The extreme beliefs and personality traits caused by the harsh nature of life slowly fade away, until the spirit has come full circle and is ready to re-enter the world. It is not a totally new start – that would be pointless – but it is good enough.
The spirit descends through meaningless space, trying to recall what it was trying to recall. This is a difficult task, for the memories in question are ancient, even predating the spirit's corporeal death, but the being is well-practiced at such things by now. Finally, the spirit is rewarded with a flicker of memory, so old as to seem primeval. It finally realizes its purpose, conceived long ago, forgotten all these eons.
The spirit has mastered the skills of time travel, of course, and of viewing the world as if through a pane of glass. With a bit more effort, it is even capable of slightly altering events. That is a trick it has never passed on to anyone; frankly, the spirit isn't certain it is supposed to possess such an ability. It learned quickly enough that it cannot change anything about its own past life, or the life of anyone close to it. Even certain events in the lives of complete strangers couldn't be touched. But there is a promise it had made, a long time ago, and it will keep that promise.
Now its target is coming into view . . . the child. Slowly, the spirit casts its view in ever-widening circles, until it finally lands upon the event in question.
Two men face each other on a crowded street. Once friends, they were now hated enemies. This is the collision that would send one of them to a terrible place, and send the other man into hiding in plain sight for decades.
The spirit ripples, disconcerted. This event is much too late; it cannot hope to bring about the change it wanted here. It has to start earlier.
As it falls back through time, it works on both men, influencing events to make the one softer, to turn him from the dark nature that would otherwise consume him, and to bring the other more understanding, and the ability to help his companion when necessary. As it works, the events in the future slowly change to fit the new past. Five years back, and the clash results in a destroyed friendship. Ten years back, and the clash never happens at all. Twenty five years back, and the men remain close for the rest of their lives.
The spirit is fading quickly by this point; it has put too much of itself into its endeavor. Briefly, it wonders if it will get to move on when it was done, or if it will just fade into nothing, completely spent. The memory of the child brings it back to task again. It shakes off the irrelevant thoughts and continues on.
The child is still an orphan; nothing could have changed that. But this time, events will follow a different path. In a burst of effort, the ghost visits the men in the time of the child, altering events to place a singular idea in their heads.
Its work is almost done. Burning its very essence, it visits another child, this one not quite an orphan, but with a caregiver wizened with age. The spirit whispers one last idea into the old human's mind before it collapses into itself.
Utterly finished, the being falls back into its own time. Never before has a mortal spirit made such a comprehensive change to the fabric of creation. The spirit knows that by the time it recovers its lost energy – if it recovers at all – it will have lost all memory of this place for good. It bids the child a silent farewell, with hope that its life will be better in the future.
Far away, a man who once made a mistake . . . chooses more wisely.
"No," he breathes, holding the child before the ruins of a house. "You will certainly not end up with the Dursleys. Voldemort is gone, though not destroyed, and you would not be safe in your uncle's household anyway. It is safest if I care for you myself, I should think." The man turns when his companion snorts. "Do you have an objection, Gellert?" he asks mildly.
"Hardly," answers Gellert Grindelwald, the well-known civil rights activist and private Auror trainer. "I simply found it hard to believe that you honestly considered any other alternative, Albus."
A/N: Yup, Harry and Neville are raised together by Dumbledore and a reformed Gellert Grindelwald. This was trickier than I had expected it to be. I'm writing this story in present tense, but a significant number of phrases here refer to past events and so needed to be in past tense. There were several cases where I just wasn't sure which tense to use, so when I doubt I used the present. I also tried to lead the reader to believe that it would be Sirius and Peter raising Harry before I revealed the truth at the end; I'm not sure how effective that was. This story does have implied Grindeldore. Their relationship may eventually be stated outright, but there will be no old man smut. Sorry, old dudes! I am just not interested in writing that. As a final note, if you want some idea of where I am likely to go with this, you can check out an explanation of my thoughts on several HP topics on my author page. There are no spoilers there.