Timeline:
Begins: Before Hogwarts, possibly 1986-87.

Pairings:
Mostly Gen-Fic.

Acknowledgements:
Some concepts here are borrowed from other fiction.
The 'San Graal' being an artefact of power made by Merlin from Stargate SG-1
The 'Mantle of Power' being passed down through Knights from The Dresden Files.
The mass Memory Charm 'Wizard's Web' from Terry Goodkind's Sword of Truth.
The 'Mind's Eye' loosely based on 'The Intersect' from a TV show called 'Chuck'.

Similarities withstanding...

Crossover:
No.

Alterations from Canon:
Pureblood ideology is still mostly useless dogma but it is true that family lines of magicals accumulate magical aptitudes to arts and gifts that they have dedicated themselves to over centuries and certain bloodlines see recessive traits with prodigies of a particular discipline of magic being born every so often. These talents and domains of superior magical skill and knowledge are more significant to the power structure, politics and economy than even elected office, appointed titles or any other such things.

Arthurian lore is, though clouded by historical revision and mythologised, the true history of Magical England and there exists the remains of that fallen empire in the magic of the current civilization.

The world is greater and more mysterious than it seems: a path suddenly opening in the forest, a column of dusk sunlight, a tremendous storm, the placid mirror surface of a large still body of water, a fairy mound, a light deep in a cavern - all ways to other realms and otherly places where dwell different races and different magics. Wizards are far from alone in the world.

Story:
Eight-year-old Harry is chased into a drainage culvert by Dudley and his 'Harry hunting' buddies/bullies where they torment him. Held under the stagnant filthy water, urinated on by one of them, before Piers Polkiss tips kerosene over him and lights it on fire and only by rolling in the water does he save himself. When Piers says they should drag him out of the water and try again Harry's panic-driven magic erupts out of him in a wave that sweeps over the gang and leave them standing about with no memory of how or why they are there.

After they wander off Harry, stunned by what he just listened to, as the thugs talk as if he's not lying right there in front of them, hauls himself out of the drain but passes out from the pain. Later that evening he limps home to Privet Drive, sunburned from hours unconscious in the sun, filth and injured he is hauled through the door and thrown to the ground by an enraged Vernon who yells about what the neighbours must think of Harry showing up at their door looking as he does.

When Dudley is dispatched upstairs to fetch the Smeltings Stick the same feeling rises in Harry that did in that drainage culvert and desperate to spare himself the additional beating he grasps it and unleashes it. Again he finds himself invisible in the sight of would be abusers who can't quite seem to recall why they were standing about as they were. Harry treats his burns and wounds, steals some of the left-over dinner and climbs into his cupboard falling asleep as he wonders what this power could be.

Over the next few days the Dursley slowly grow more aware and more antagonistic telling Harry that whatever it was is wearing off. He knows this must be 'freakishness', his Uncle and Aunt's tirades and punishments made that only too clear, but for the first time ever he actually starts to wonder what it even means. Reasoning it out he comes to the conclusion that he must be different from everyone else, the Dursleys know it too because they alone blame him if something strange happens when he is around. They didn't always know something had happened, not unless they saw it.

Harry uses the last of his 'ignore Harry' respite to skive off and try to practice his ability. Only by concentrating hard, making himself feel anxious, afraid or scared can he get that sensation back. Once he has it though he can try to pull on it, he just has to give it something to do. He makes a pebble disappear out of one hand and appear in the other. His attempt to repeat the 'forget Harry' trick doesn't work however, but the dirty dishes he is scrubbing when he tries it all suddenly become clean.

He can make thorns fall off his Aunt's roses, his cupboard under the stairs is as warm or cool as he likes, the blunt knife glides through his Aunt's breakfast grapefruit, spots on windows and laundry disappear, he can make them ignore the extra food he dishes onto his own plate and with practice on the sliding bolt on the outside of his cupboard door he figures out how to make it open from within. Easier than 'forget Harry' was 'don't look' and all he had to do was find a hiding spot and make sure the first one of Dudley's gang to come didn't look at him and he was left alone.

Leaves would float in his hand, drops of water freezing into chips of ice, making a match catch alight, making the heavy watering can feel lighter, calling a pebble to roll across the ground towards him and even leap up into his open hand. 'Don't look' turns into 'don't notice' and while the Dursleys know he is there and can speak to him and look at him when they really want to the rest of the time they ignore him, especially Dudley who seems somewhat confused by not punching or kicking Harry whenever their in walking to school, in class or on the playground.

Everything is looking up for Harry... until the day Dudley comes down the stairs, gawps at Harry and asks who the hell he is and why he's in their house. The magic used on Dudley has accumulated to the point he doesn't even remember Harry, period. No 'don't notice me' is going to stop Vernon Dursley's swift and totally correct conclusion that this is freakishness worked on his son by the Potter freak. Harry is backhanded, kicked and runs into the street where Vernon catches up with him and starts strangling him right in front of wide-eyed neighbours.

As the darkness closes in, Vernon's bellows of rage and the shouts of neighbours filling his ears, Harry's magic lashes out and in a burst of unprecedented magic he weaves an entirely new spell around himself. 'Forget Harry' and 'don't notice me' weave together and now nobody remembers him. Not the Dursley's, not neighbours or teachers, not Arabella Figg or Albus Dumbledore. While the Wizarding World might remember 'the Boy Who Lived' if you asked them who Harry Potter was they wouldn't even hear the question.

Harry Goes Free:
Seeing no reason to stick around at Privet Drive he hops the bus into London and starts his life as a virtual ghost. Movies, museums, buffet restaurants, libraries, tourist destinations, some shoplifting but never greedy, sleeping in any hotel where he can reach the room keys behind the counter. Not being seen does get to be a bit isolating but experimentation with his magic lets him figure out how to draw someone's attention and make them remember him if he wants them to be able to do so.

One evening while getting off the Tube he sees a man grab a young girl and carry her into an alleyway as she kicks, thrashes and tries to scream. Harry saves the day, a magic amplified punch to his leg, leaping onto his back and gouging at the man's eyes causes the creep to let the girl go. When the man tries to grapple with Harry he starts pounding 'go to sleep' into the man though the skin contact of the man's hands around his throat... and the girl rushes up and punches the man in the groin.

A very grateful Hermione helps a winded and bruised Harry to his feet and leads him to her parent's dental surgery. She worries about leaving the creep behind but Harry just hands her the man's wallet and tells her she can give it to the police. When she goes into the office to get her parents to come look at Harry she comes back to find him gone and the receptionist doesn't even seem to remember seeing him.

Harry becomes a bit of a vigilante after that, even 'studying' comic books for ideas on how to use his power with limited success but success nonetheless. Police get an awfully large number of anonymous tips on their hotlines and Harry saves more than one working girl from her pimp. Eventually he decides to take a holiday (and his vigilante act on tour) and starts hoping buses and trains all over the UK. Fairs, theme parks, tours a chocolate factory three times, several castle ruins, takes in some football matches, Harry gets around.

Everything changes though when wandering down the street in Coventry he feels something pulling at him. Wandering into the standing remains of the old Cathedral he sees a transparent illuminated figure... an actual ghost... and its looking right at him! The ghost says his name is Sir Jonathan Ashwell and he is, was, a wizard just like young Harry. He was killed in November 1940. No, not by the German air raid, by soldiers working for a dark wizard named Grindelwald. The air raid was cover and the firestorm was magical, a trap laid for Ashwell. The ghost waves his hand through the stone base of the old pulpit and out of it, like something bobbing up on the surface of a pond, comes a signet ring.

As Ashwell was dying all those years ago he took it off and activated the spell upon it to seek out a new wearer but the German wizards' wards on the town to prevent Ashwell's escape also held the ring in place and to make sure they didn't get it he hid it in cathedral. He has waited as a ghost ever since for another magical to come and take it up, to take Ashwell's place... as the Knight of Albion.

Sir Harry:
The tale of the San Graal in Arthurian mythology was conflated with the Holy Grail by Christian historians many centuries later. In actuality it was the repository of all Merlin's knowledge and the vessel for his power, created by the wise sorcerer as he reached the end of his life. He built it and stored it in a refuge called the Siege Perilous, upon the isle of Avalon centred in a loch which is itself (during the right alignment of the sun and moon) a gateway to the realms of the beings who were worshipped by the Celts, Welsh, the Irish and Druids.

The Knights of the Round Table, most of them magicals of one degree or another, submitted themselves to the judgement of the San Graal but only Sir Galahad (not Percival as other later mythologies claimed) was found worthy. Upon his mortal death he became its eternal guardian, his soul incarnated materially but unable to leave Avalon. Ever since then Merlin's mantle has been passed down from magical to magical. If the ring Merlin forged judges them worthy it transports them to Seige Perilous for Galahad to train. With the mantle comes the wisdom and knowledge of Merlin and every holder of the mantle preceding, the 'Mind's Eye'.

Harry is found worthy by the ring and whisked away to spend the next 2 years being trained by Sir Galahad to become the vessel of Merlin's power and wisdom, to defend the realm against its enemies, to bring justice to those who suffer injustice and to punish wickedness... the corrupt, inept and indolent Ministry and populace are in for a rude wake up call.

Before his 11th birthday Harry breaks the spell upon himself that hides all knowledge of him from the notice and recollection of the world, to ensure his enrollment in Hogwarts occurs. The Harry the magical world is about to meet will make the Boy Who Lived look like the fairy tale he is.

Probable:
Dumbledore and Snape bashing as they are confronted with a Harry above manipulation and immune to mind-magics but nevertheless determined to have control over what they believe is their means to an end.

Merlin had many legacies meaning many duties beyond the role of a knight. He brokered peace agreements between magical being of many other realms and mankind, such as with the Unseelie a.k.a. the Fae Folk, and it remains the duty of the possessor of the Mantle to act as the diplomat and enforcer of those agreements.

There are evils out there worse than Voldemort and eventually Harry may be pitted against them as well.