on harry potter's seventh birthday he regained his memories as the former dark lord gellert grindelwald and all his powers. at first he was puzzled at his situation then after figuring out that he was now living with muggles who treated him horribly and was surrounded with blood wards with a very familiar magical signature he decided he had enough of his old friend's bullshit, ripped open the wards and apperated away to diagon alley (he also broke all the tracking charms and core blocks).
after landing in a small side alley he and albus used to meet in and using human transfiguration to look like a eleven year old muggle-born on his first time to diagon alley, harry started walking towards grinngots.
entering grinngots harry walked to the nearest goblin and said in goblin speak "greetings great goblin i'd like to talk to the account manager of the potter, peverell and ...grindelwald vaults", the goblin shivered a bit at the implications of the last house mentioned "of course, fallow me".
the goblin lead harry to a room with the potter, peverell and grindelwald vaults "hello there great goblins, ironclaw" he said looking towards the grindelwald secret vault account manager "who are you?" the peverell account manager asked "gellert grindelwald and harry potter" was the former dark lords only response before using human transfiguration to make himself look like he did in his prime making all the goblins in the room flinch and a smile to creep apon his face. o boy was this going to be fun.
albus dumbeldore was currently a very worried man. the reason is of course a certain boy-who-lived or to be more accurate his disappearance. early this morning all magics around number 4 purvet drive were ripped open and harry was apperated away, even the tracking charms were destroyed leaving no leads at all. he already sent a notice to the DML to find the boy-who-lived, they then presided to send every auror available to search. albus could only hope they'll find him before tom's followers, and if not then merlin save them all.
after swearing the goblins to secrecy and making a few (a lot) of deals like giving him the hiership tests to the potter, peverell and grindelwald lines (he managed them all) and a few others, he used human transfiguration to make himself look like a twenty years old gellert and left the bank with three sacks connected to his trust vaults, he then started a giant shopping spree.
first harry went to the best trunk store he could find and bought a trunk with 8 compartments made out of cherry wood and dragon skin and heavily charmed, he then bought about two hundred books (mainly runes, arithmancy and recent history) from all across diagon alley and five tons of potion materials.
after he finished buying things in diagon alley he entered knockturn alley and bought himself the most agreeable (has the lowest resistance) untraceable wand (alder 12 inches, dragon heartstring) he could find, he then bought two tons of wand materials and about eighty books about the dark arts and the unforgivables.
he then bought five house elves and sent them to buy allot of food and drink. after doing all that harry apperated to his (grindelwald's) hidden base, the shadow-tower and raised the wards to high heaven then started preparing everything he needed ...the green light in made dreams is probably the killing curse and i have a feeling the one who did it isn't dead so i guess it's time to try to find a way to counter it again ha... harry thought to himself as he set up a gigantic arrangement of books about runes, arithmancy, the dark arts and the unforgivables in front of him ...and a custom wand...
three years time skip.
harry was currently sitting in the shadow-tower's library his new wand (blackthorn and vine wood, 11 inches with dragon heartstring and thestral tail hair core, very good with battle magic and runes) hovering over pages upon pages of arithmetic and runic theory and calculations. "maybe like this... no wait this, yea letts try this" harry said as he stood up and apperated to his training room were mage battle golems awaited, "one opponent, available spells : avada kedavra" he shouted as he took a defensive stance ...thank god you can't actually die here because of the wards or i'd never be able to test this... harry thought to himself as one golem stepped forward and shot the killing curse at him, harry countered by moving his wand in the shape of the symbol of the deathly hallows at shouting "reflecto maxiam".
as the two spells crashed against each other the killing curse seemed to suck in the shield, become twice as big then shot back towards the golem destroying it. "yes it worked" shouted harry now full of joy ...three years of work finally paid off... he thought then started experimenting.
five hours later and harry had relative mastery other the spell he also found out the spell doesn't only reflect the the enemy dark charm but also doubles the power of the spell. all in all it was neat, it's only real weakness was that it only worked against dark charms (meaning it doesn't stop stupefy, expelliurmus and the like) but he already mastered both protgo and portago maxiam as gellert grindelwald so that didn't matter much.
one year time skip (i'll be calling harry gellert from now on)
gellert was currently sitting in the library of the shadow-tower, he was in the process of writing an arithmetic formula when he suddenly felt a owl enter the wards ...the hell... he thought to himself as the owl handed him a letter, it read ; harry james potter-the shadowtower-main library-the land of shadows. ...an automatic one then... gellert thought as he opened the letter.
after reading the letter gellert was in deep thought, on one hand he really didn't need any magical education, hell he is probably one hundred times stronger then all the teachers (excluding albus) together, on the other hand however it's the simplest why to reintroduce himself to the magical world (not that he was planning to do anything big until he was at least twenty) and easiest why to gain loyal followers early on so in the end the latter won and he decided to go to hogwarts.
after he finished preparing gellert apperated to diagon alley and bought the first year school supplies with a few additions such as 'hogwarts : a history' and 'basic occelamancy' (the second one was so he'd have a easy excuse for knowing of the art).
he then went to olivanders and bought himself a wand (for cover of his real one) which ended up being voldemort's brother wand. when he finished gellert apperated to the shadow tower and prepared his things.
one month time skip
gellert stood before the hogwarts express and felt sober for the first time since his awakening, he was actually going to hogwarts, he was going to meet albus again and he was going to build himself up to be the next dark lord without anyone being the wiser. and as gellert entered the express abus dumbeldore was hit with several danger alarms from the express.
author note; gellert can use human transfiguration like a metamorphmagus.