Power the Dark Lord knows not
Lilli and James use dark forgotten magic to give their magic to Harry upon death.
That was why they didn't fight Voldemort.
A witch and wizard, caught without their wands? Dying without fighting with their magic? Unlikely.
When Voldemort tries to kill Harry, unexpectedly, the spell claims his magic as well. That's why he loses his body, because wixen don't survive having their magic excised.
Every time Harry kills something magic he absorbs their magic as well.
His magic protects him from those who attempt to kill him by taking their magic as well.
He is tainted by the dark ritual that was sealed with human sacrifice.
1st year: The troll, Quirrel and what magic Voldemort had gained back by drinking unicorn blood
2nd year: acromantula, the basilisk
3rd year: horde of dementors
4th year: Voldemort again??
5th year: more dementors in little whinging, Voldemort again in the DoM??
6th year: inferi (sort of, not alive, but imbued with magic)
7th year: Voldemort again during the battle of the 7 Potters, Nagini at Godric's Hollow, absorbs the fiendfyre, Voldemort final time.
Voldemort is perhaps able to pull back when his magic is pulled at after the first time? So he doesn't lose his body again every time?
Magical creatures tend to sense Harry's magic and leave him alone, unless dumb (troll), gluttonous (dementors and some acromantula), controlled (basilisk and nagini).
Centaurs cryptically hint at his power, goblins are wary of him?, house elves are in awe?
probably fluffy cowers from him, aragog probably feels it but can't control all his brood, basilisk might shy away and fight riddles control, crookshanks knows, hippogriffs respect him, boggarts fear him, werewolf moony runs away, dragon absolutely knows and stands back, pretty much everything in the lake leaves him alone, merepeople run away, sphinx?, thestrals respect and like him?
Harry is visibly marked by his immense and growing power. He heals almost instantly which makes the Dursleys fear him more. His hair moves as if in a breeze, like flickering flames, his eyes glow and flash like burning coals and lightning.
Sometimes his voice reverberates?
These grow and take on aspects of the magic he has absorbed.
He is more feared by the wizarding world. Some people can sense his power subconsciously.
After freaking them out one too many times, dursleys leave him at an orphanage in the night.
Not able to find any record of him or his family, they give him a new identity.
Hades Black. Named because his unsettling appearance and presence reminded the carers of a demon.
Stumbled upon by wizards? Lucretia Prewett née Black (60+),widow, aunt to Sirius, childless. Cassiopeia Black (70+), never married, great aunt to the Black sisters and brothers, sister to Dorea Potter née Black.
Dumbledore suspects he's Harry, but says he is safer with his identity hidden from the wizarding world and from himself.
In this fic instead of Euphemia and Fleamont Potter being James’ parents, we’ll say Dorea and Charlus were. So Hades is grandson to Dorea Black, great nephew to Cassiopeia.
Cassiopeia’s squib brother Marius ran the orphanage he was left at and was the one to name him Hades Black and took him in as his own as the other children and carers were scared of him. But he recognized the magic. Marius contacted his estranged sister. They guessed his likely identity.
As he is a Black by blood, and most of the younger generations are either dead, in prison, or raised and shaped by other families and traditions, the Black cousins take him in as a chance to shape him in the image of house Black and revive keep their family alive.