Disclaimer:I do not own Harry Potter.
Not sure where this idea came from, but it would make for an interesting story.
When Harry Potter was seven, Dudley Dursley tried to drown him in the playpool out the back of the house.
Water is a connective element and Harry was already a very unhappy boy, but also a very powerful wizard.
Those of the magical community had to be very careful of their children, and muggleborns were carefully monitored, because sometimes, in a moment of weakness or desperation, the magical child can accidentally call out and…
…let something in.
Some theorise that this is what happened to the one called Voldemort.
It was also something Dumbledore, in all his finite wisdom, forgot to ward against. Not even the alarms went off.
Harry was…lucky in a way, that he was stronger than the Vengeance Demon that slipped in through the small crack in Harry's defences that the larger entities vying for possession of the boy could not.
It was a nameless creature, too young and weak to yet be strong enough for more than basic instinct, and so, upon possessing young Harry and merging with the child's mind, the demon assimilated Harry Potter's memories, in the confusion assuming that it was Harry Potter itself.
And so the symbiotic demon became a darker aspect of Harry, thinking and acting and influencing in sync with the powerful young wizard, who became angry and vengeful against much of the world that left him to the Dursleys and offered no help.
Things began to happen to those that wronged the boy or saw the abuse but did nothing. Small things, large things, things that there was no way for a small boy of eight could have influenced.
Mr Cane in Number 2 lost his job because of budget cuts.
Ms Goldstein in Number 7 forgot to put the hand break on in her car and it rolled over Mrs Cassidy of Number 6's little Maltese Terrier, Sukie.
Mr McDonard in Number 1 came home early to find Mrs McDonard in bed with the twenty-five-year-old fiancé of Miss Troy in Number 11.
Mrs Figg lost seven cats before she found the large Gaboon Viper under her house that had escaped from a house on the next block. Because the snake had been illegal, it was not reported missing and the owner was not found until after Mrs Figg was taken to the hospital and in intensive care from the venom, having tried to save one of her cats from the snake. Mr Tibbles died anyway.
Aunt Marge tripped over Ripper on the stairs and broke her upper spine, rendering her a quadriplegic. Ripper had to be put down and further investigations into the dogs reveal mistreatment of the breeding bitches and illegal disposal of puppies deformed from inbreeding.
Dudley Dursley developed some rather severe allergies to chocolate, peanuts, several common preservatives and some of the more commonly used artificial colourings. After Dudley went into anaphylactic shock and was rushed to the hospital for the seventh time in one month, Child Services removed both children from the house pending investigation.
In the resulting investigation spanning two years, the Dursleys were sentenced to prison for a variety of child related, forgery, larceny and embezzlement charges.
Most of the staff at the school where the children attended were investigated and found to have failed to notice or report abuse, suspected abuse and neglect. Most of the staff had had their licences revoked.
Seventeen reports of suspected abuse and neglect in regards to the minor Harry James Potter had been found in the Child Services database. All those involved had cut off their investigations and left their line of work with no warning. None could recall a boy named Harry Potter. Foul play was suspected when six of the thirty-seven caseworkers, school nurses and police officers displayed erratic behaviour ranging from confusion to memory lapses to short periods of catatonia when questioned about the boy and why they left their previous jobs, which most of them had been passionate about.
Dudley wound up in an institution for children and youths with mental disorders after being diagnosed as a psychotic sociopath.
As for the now nine-year-old Harry Potter, he was bounced from foster home to foster home, orphanage to orphanage, case-worker to case-worker, as a line of accidents and misfortunes followed him, putting three paedophiles, two rapists of teenage foster girls, a ring of child-slavers and a number of abusive carers and negligent case-workers in jail or on trial.
There was a complete overhaul of the system when the media caught wind of young Harry's 'ordeal'.
No one suspected that the demon, and so Harry, could sense those against whom Vengeance was demanded by the wronged, and to what extent, and subconsciously manipulated events so as to be more able to go after these 'victims' on which the demon preyed to gain power.
The day Harry Potter was re-introduced to the Wizarding World, he passed seven Death Eaters, fifteen serial rapists, a potioneer who like to experiment on non-human sentient beings and muggles, several malicious gossips and a whole range of various petty evil-doers and politicians.
It was a busy week for St Mungos, Azkahban and The Morgue.
Harry's powers spiked dramatically and continued to climb to the point where other demon-possessed beings were giving the demonic boy a wide berth.
When he arrived at the school, several of Harry's year-mates nearly drowned when their boat tipped and the squid wanted to 'play' with them. They were all sorted into Slytherin.
So was Harry.
By the next morning, there was an influx of aurors and healers coming and going from the school due to the large amounts of accidents that had happened overnight, as well as the fourteen suicides and the fatal potions accident that took the life of Potions Master Severus Snape when he came upon two red-headed twins brewing a potion, using his body as a shield to protect the twins and dying a hero when it became unstable. The Weasley twins never pranked again.
Those who knew what happened to Professor Quirinus Quirrel tried valiantly to repress their memories of his agonising and gory death as he was caught between two of the moving staircases, the unicorn blood drawing out what would have been a relatively quick death.
That fate of Albus Dumbledore was ever after only whisper of under bright light and protective talismans. And never, ever after dark.
Harry Potter went on to be the greatest head of the DMLE in the history of the Magical World and only Luna Potter nee Lovegood and their children ever saw the brilliantly beautiful green scales and barbed frill of his true form.
The Vengeance Demon continued until the end of Creation as 'The Harrier', Demon Lord of Vengence and Just Hand of the Wronged.