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Albus Dumbledore had been given full reign of the Black Family Library while they were using Grimmauld Place as a base, Sirius had no fear that he would misuse the knowledge and knew that the older man may find some useful information in the books that could help them to defeat Voldemort for good.
It was in one of these books that he stumbled across a spell that would guarantee a person full-time protection from what the book called a guardian, so long as that person had not reached maturity and was an orphan.
This guardian was some kind of creature that would follow the person around, deflecting spells sent against the person it was protecting, and attacking those who became too violent for its liking.
Of course, it was a very old spell, and he'd had to translate it himself, there was likely some inaccuracies in what he understood it to be, but it couldn't be too different from what he'd uncovered.
So, two days after the boy had been relocated to Grimmauld Place, he had him follow him to another room, away from the rest of the Order of the Phoenix, they couldn't know what was going on, as the spell required the physical harm of the child gaining a guardian, something that Albus really didn't feel comfortable doing, but would push that discomfort away if it meant that Harry would be safe, for at least a little longer.
Harry looked at him curiously as he led him into an unused office, three floors and multiple silencing wards away from the dining room where everyone else was.
"What was it that you needed me to see, Headmaster?" he questioned, and Albus had to look away from him as he stared at him with such trust.
"I am sorry, my boy," he muttered, steeling himself, he whipped his wand forward, sending him flying backwards into the wall with a low powered Bombarda.
He smacked into it with a crack, and Albus quickly started chanting the long-forgotten spell as Harry cradled his now broken arm and crawled into a corner, trying to hide from him.
He ignored the slight whimpers he heard from the teenager, shutting his eyes as he came to the final words of the spell, a surge of magic filling up the room as Harry began crying and a guardian was formed.
Except he was wrong, it wasn't a spell to create a guardian.
Albus found himself thrown back onto the ground, groaning, he opened his eyes and came face to face with an unbelievably angry blond man in a hoodie who'd scooped Harry into his arms.
Harry had stopped crying and was staring at the man as though in a trance, and Albus now realised exactly what the spell did.
He could feel the bonding magic strong in the air, something that only came with decades of learning to feel it, and as the man turned to look at Harry, it snapped into place.
The man stood up, pulling Harry with him and glared at Albus, before walking out of the room, gently leading the Boy-Who-Lived away.
He only got over his shock an hour later when he stumbled into the dining room where the Order was waiting for him with panic on their faces.
The man had walked out of the house with Harry in tow.
Not even a month later, Albus received seven destroyed Horcruxes, one of which was shoved in a tiny stick with a note mentioning that it had indeed been in Harry's scar, and Voldemort's decapitated head in the post.
He supposed 'guardian' was pretty close to 'violently overprotective magic chosen parent' in the end.
Funny enough, Albus decided that he'd much rather deal with Voldemort for another seventy years than the Master which was what the man called himself.
He was much better at taking over the Wizarding World than Tom had been and having Harry at his side helping him was even worse.
Of course, his opinion on the man wasn't helped by the fact that he was hunting him down for the harm he'd bought upon the boy when he'd summoned him, apparently the man wanted his head.
Maybe he should have left the spell alone.