Harry was bored he had been bored for a long time now it all started just after he had defeated Voldemort only to get a killing curse to the back of the head. But not die. Not even pass out. For a single timeless moment, everything on the battlefield stopped for what felt like an eternity but was only a moment.
Suddenly! He serged into action, with a sectumsempra the unlucky Death-Eater that tried to kill him was without his head. As if a gun had been shot and the race started the battlefield came back to life, spells flying everywhere, Harry in the middle of it dropping death-eaters one after the other like a force of nature tearing through the battlefield.
After 30 minutes it was all over the death-eaters were all dead captured or had long since run away.
That had 1000 years ago, it turned out that when he had picked up the last of the three deathly hollows he became The Master of Death. A fact that he was obvious to till he was berried under a building in one of the last retaliations of the leftover death-eaters only for once the rubble was removed he was alive and walked away with only a few broken bones. Now 1000 years later still looking just like he did when he picked the last Deathly Hollow up. All his friends and family had long since died and his now grand, grand grandkids, numbered so many that most did not even know that they were related. The Magic world had changed with them now an open part of day to day thanks to a lifetime of work from his best friend Hermione and other school mates the only things about his day around are old books and legends of what he and his friends did.
He had tried going on without his friends grieving their death for years in isolation learning the full extent of his immortality in his darkest hours when he had tried everything he could think of, even going as far as to walk through the archway of death inside the ministry but nothing worked cuts healed, poisons where flushed out not that they even worked with the basilisk venom in his blood, cutting himself to pieces just left him with the limb reforming not even decapitation worked his body being completely reformed and working once he woke up, not even one desperate attempt to atomise his body worked his body just slowly reforming it taking over a year to do so but in the end, he was left alive still. Driven more then a little crazy from depression and the inability to die he is hit by the idea if he can join them in the afterlife he will join them in the past but he did not just want to go back and relive life as before if he was going to relive everything again he was going to enjoy himself.
It had taken him 200 years to get to this point. 200 years to make the tools and ritual needed to complete his goal, the three main parts needed to go back, to when his friends were still alive, were a potion that took nearly a year to develop and nearly 30 tries to get perfect.
The potion once drank would kill anyone who drank it, if somehow you can get past that part it turns your body into pure magical energy which is what kills anyone who drinks it also, in turn, creating an explosion on the level of a nuke but if you could live through it enables you to use all that magic to do otherwise impossible things.
The second part took the longest as it was more the skills and knowledge needed it took 50 years to become a master blacksmith and gem cutter trained under the goblins him long having made up with them by returning a large number of goblin artefacts over the years, 30 years to become a master wandmaker and finally 60 years to master runes. It then took another 20 years of study and development to make the tools needed a diamond rune inscriber itself covered in 10,000 of runes to help keep perfect control of the hand and keep it steady able to make the perfect lines needed.
Next was his wand a perfect match it is 11 inches long dual wood half Elder soaked in Phoenix tears freely given and half Holly soaked in basilisk blood and venom took freely given by a basilisk he had found dying alone and in pain staying with it for weeks as it slowly succumbs to its wounds.
The base of the wand there was an emerald carved by hand and magic covered in thousands of tiny runes, the core of the wand is a Phoenix feather from Fawkes covered in a mixture of his blood which had basilisk venom and the ash of his old wand.
The reaction when he first tried some magic with the new wand turned a simple accio spell for a book across the room into a massive feat of magic as instead of the book flying to his hand as it should have it teleported to it and not only the book he wanted but ever book with a mile radius was teleported into the room, lucky he was far away from anyone else but unluckily he had now been collecting books for almost 1000 years and had long since taken possession of Hogwarts library and many other great collections of knowledge over the years this left him buried under literal tons of book that had been teleported into the room-filling from floor to ceiling and crushing Harry till he was able to control his magic again and undo the spell sending the books back to where they had come from.
The final part is less of a part more a place and room the place being Hogwarts and the room is the room of requirements he needed it for the magic the saturated the place from the crossing Layline it sat on and the leftover magic from hundreds of thousands if not millions of children who had passed through its halls over it's 1500 plus years being used as a school before finally being retired after magic having been so long integrated into normal life that there was no difference between normal and magical schools.
The room of requirements could give him the perfect size room down to the millimetre which, when dealing with Magic of extreme complexity can and does affect everything. The next 9 years were spent getting everything in order in the outside world and finally, his last few years have been spent in the room carving millions of tiny perfect runes into the ground, roof and walls with no rest long having learned that his bodily needs where now mental wants not physical needs his physical body and magic unaffected by there absence.
Now 200 years after first starting to plan he finally stands ready to drink the potion and send himself back in time and merge with his younger self, with a few fun added extras.