Disclaimer: I do not own Harry Potter or any Norse Mythology. I am not old enough for the later.
This is basically what would happen if Lockhart managed to caste the memory charm in Chamber of Secret on Harry instead of himself. The rockfall killed Ron and badly injured Lockhart. Ginny, of course, died while Tom came out of the Diary.
"So I'll remember…even if I forget."
Harry could only ever remember the white room he lived in. It was filled with puzzles and white flowers (lilies, part of him whispers) and a pale blue stuffed rabbit that sits on the bed with him.
Sometimes he doesn't even remember his name or how to speak or move properly.
Something is wrong with his mind, the lady in white who comes into the room often tells him. A miscast spell has hurt his ability to remember.
Sometimes he vaguely remembers the old man who came to visit him at the beginning, when his mind and soul were still hiding. He would pry Harry's eyes open to dig and claw and tear at Harry's mind.
It only made him hide deeper.
Another man used to come, and still does.
He is tall and dark (and so sad). Usually he just stands at Harry's door and looks in, but sometimes he will enter and sit by Harry's bed when he thinks Harry is asleep. Harry thinks it was this man who left Harry his rabbit.
A ragged man came once; clad in drab greys (bringing a sense of cold loneliness). Harry gave this man an odd stick from beside his bed that gave him a sense of warmth, telling the man "Be free…" without knowing why, just that it was the right thing to do. The man had cried, but left eventually.
Harry remembers the girl with the hair like a blackberry bush, tangled and big. She visited once in the beginning (the little girl with tears like shattered dreams and broken friendships) and once later (the gaunt young woman with dry eyes and tatter heart).
He sometimes feels that there should have been a person with red hair, but usually dismisses it. (His mind shies away from that memory.)
There was a time long ago when there was yelling and screaming and bright colours as people in black robes and white masks moved beyond the room.
Tom came after that.
Tom the Riddle would come in and watch him sometimes, a smile on his handsome young-old face, red-brown eyes gleaming with indulgent, smug mirth.
His cold hands would sometimes cradle Harry's jaw, tilting it up so he could look into Harry's child-like green eyes, slipping into Harry's shattered thoughts like a serpent through a bramble bush to meet the smaller serpent that lives at the centre with the mangled child that is Harry.
Tom's touch in Harry's mind is like slick, smooth water, soothing the jagged edges the old man and the spell damage left behind, compelling the boy to lean into the hand cradling his face. In a rare lucid moment that occurs when Tom brushes against a snare, Harry wonders if he could have loved Tom if he weren't so broken and un-whole.
As this thought filters through, Harry can see the smile slide from Toms face to something like regret.
But the thought is lost in the haze of a broken mind, drifting into the snowstorm that rages outside the bramble of his mind.
At some point, a vague memory of a picture floats past; a picture of a girl and a wolf and a giant snake that were the children of Loki.
Harry wonders if he is the half-corpse Hela, for surely he is neither Fenrisúlfr nor Jörmungandr.
Tom finds the thought amusing and begins calling Harry his 'Hel'.
It is while Tom is gone that the old man returns and hits Harry with a red light.
When he wakes again, the world is loud and garbled and confusing, with strange people touching him.
It is like the trauma of being born again and, like a newborn, Harry screams and wails and flails until the red light returns.
So little 'Hel' ran back into the bramble of his mind and into the safety of Jörmungandr's coils as ice freezes over the outside, burying the bramble in snow.
Harry doesn't know it, but a new, false personality is imprinted over the snow above the bramble while he is trapped with the Serpent, waiting for the Wolf.
And Hel puts himself back together, greenery and berries and flowers of thoughts sprout and bloom and hang heavy from the buried boughs.
The cold place becomes warm and the serpent observes and eventually alters things as Hel works.
He manages to put himself together enough that he can think and feel and sometimes remember, but Jörmungandr keeps some of Hel's memories deep within his coils where Hel could not reach, even if he wanted to.
He remembers Harry Potter now, but it is distant and unwanted. Harry Potter was an unhappy person buried under unwanted obligations he would never have chosen for himself.
Hel is reasonably content and, while not happy, is better off than Harry Potter, so Harry Potter is separated and put to sleep within Jörmungandr's coils.
Hel gathers all the magic within them and puts it to sleep before settling down to wait for the God-Slaying Wolf with the Midgard Serpent.
In the outside world, Harry Potter, with the personality of Ronald Weasley imprinted over the top, suddenly finds himself unable to call forth the Magic that lays still and silent in his veins, ignoring the call of One-Who-Is –Not-Its-Master, during what would have been the Final Battle.
Light streams into the brambles as snow is dragged away chunk by chunk, two sets of eyes watching and waiting to see if it was the Wolf come to free them.
Hel wakes with the taste of blood on his tongue and chains on his wrists. A familiar hand lifts his jaw.
Tom the Riddle stands above him, older now, with eyes like wine. Hel smiles up at him with a child's devotion.
Tom smiles back, pleased.
Hel wakes up again in his white room with his blue bunny.
Tom and the tall dark man he thinks might be Loki in disguise still visit him.
Only now, sometimes Tom lies with him, touches him, and smiles with a wolf's smile, well pleased with his 'Hela'.
Voldemort regarded the young man cuddled up against him with the regard one has for a well-earned possession.
When Harry Potter, now Hel, had been hit with a miscast Obliviate spell by the fool Lockhart, he never expected the boy to be so fascinating, even broken as he was.
Dumbledore had tried to hide the boy's damage, saying he had been sent away to be trained. When the boy had been found in seclusion in the Janus Thackery Ward at St Mungos, his first thought was to do away with the boy, then nearly eighteen and surrounded by a tumble of black hair that stayed around his hips, even when cut.
But when the boy had looked at him with his empty childlike eyes, Voldemort had changed his mind, instead slithering into the depths of the shattered mind to find the piece of his soul he hadn't been aware that he had split off.
As a Horcrux, Harry was valuable.
Docile and affectionate, Harry's mind was more instinct and emotion with few lucid thoughts and a mangled ability to recollect short- and long-term memory. Harry vaguely remembered the Tom Riddle of the Horcrux Diary in association with an ally and a mild puppy-crush.
The mind had been easily led and moulded, Harry very close to becoming Hel, so Voldemort knew something had been done to Harry Potter when Dumbledore stole the boy on the cusp of being a man.
Then Snape had reported what had happened, honestly worried for the boy.
It was perfect timing that the puppet in Harry's body had suddenly found himself unable to carry through with the suicide spell that would have killed them both, as by that point, Harry and himself were the only Horcruxes left.
The magic was still there, as Voldemort found when the boy was captured and kneeling before his throne in the Great Hall of Hogwarts.
Then Dumbledore was thrown at his feet beside the boy.
Voldemort, as a nod towards Harry Potter's twisted view of the world with Voldemort as Fenrisúlfr, Harry himself as Hel and Dumbledore as Odin One-Eye, removed one of the old man's eyes and carried out the function of Fenrisúlfr at Ragnarok; to slay the Allfather.
It took Voldemort three days to dig out the false mind to find the bramble of Harry's mind beneath. To his surprise, Harry had continued to evolve beneath the mantle of winter and had 'fixed' himself to the point where he was again functional.
As Hel woke, the magic lying still in his veins burst to life; previously shorn hair tumbling to the knees, slight shifts occurred in facial bones and muscles to give a more feminine look and dark dapples like bruises appeared over half the boy's face.
The eye on the discoloured side turned bright red.
He truly was the appearance of Hel.
The familiar childish smile of devotion spread over the fragile face.
The Chamber of Secrets became Hel for his Hela. His Horcrux safely stored in a white room within Slytherin's statue, accessible only to himself, Severus Snape and the golem who cared for the boy.
Voldemort was pleased.
Harry doesn't have to make sense. He's crazy.