This is a triple cross between HP/Supernatural/Constantine. It's also a Castiel-is-Harry story, if you don't like, don't read.
When the angels noticed a problem, the first thing they did was send someone down to fix it. His name was Castiel.
For some reason, the birth of the Potter child wasn't going as it should have. He would be one of the many infants who died of unknown causes and there was nothing the parents could do to stop it, even if they had known there was a problem.
So Castiel descended onto the Earth, got permission from the mother to enter, and was placed inside the infant's body. The plan was that he would end the Dark Lord before he made another phylactery.
There was just one problem. The angels in charge didn't account for the effect the killing curse would have on a lowly soldier like Castiel.
The last memory Castiel would have of his true self ended the moment he felt the foul nature of the curse. A higher angel, one who had commanded others, might have been able to shrug off the human magic. But Castiel was a foot soldier, one who took orders, not give them.
The moment the curse hit, his Grace sent it flying back...but in the process his guard dropped long enough for the magic to scramble his memories.
He was as helpless as the infant he now possessed. The boy had passed long ago...the only one in the body was Castiel.
The horse faced woman dragged the child into the church. Surprisingly he put up less of a fight once inside than he ever had outside the place.
It was like once he was inside the hollowed ground, he became a different boy.
The woman hated the child. If she could, she would have dumped him on the Church's orphanage in a heartbeat. The worst part was the brat would scream at nothing!
"So who is this young lad?" asked the priest.
"My sister's son. We want to get him exorcised."
The priest blinked.
"You're Mrs. Dursley aren't you."
Her eyes glinted in a hard manner.
"Ma'am, as I told you over the phone, exorcisms are expensive. The materials alone..."
"I don't care about the costs. I want the brat to quit screaming for no reason in the middle of the night about monsters."
"Mrs. Dursley, many children are terrified of monsters," said the priest patiently.
"Yes, but how many of them will willingly barricade themselves inside a small cupboard to escape monsters?" she countered.
The priest bent down to his level.
"Son, what kind of monsters are you seeing?"
"People that aren't people. They walk around pretending to be normal, but their faces are all wrong..." said the child. The priest noted he didn't try to get anywhere near his aunt.
The priest had an idea of what the child was really seeing, but he wanted to confirm it before he called that man in. The last thing he wanted was to introduce him to a possibly abused child.
The priest took out a book and showed the child the pictures inside.
"Which ones did you see?" he asked.
Most of the pictures were fake...but the select few that weren't would tell him all he needed to know. The child looked at each carefully...before he pointed at three. The priest noted where the pictures were and paled.
The child was psychic. Most of the demons he picked out were able to come up without any trouble...and hide with perfect ease. Most children would pick out the most terrifying or striking.
Not this one. He went straight for the demons.
The priest looked at the woman, who narrowed her eyes suspiciously.
"I can help your nephew. He doesn't need an exorcism. He needs to know how to keep the things he sees away," said the priest.
The woman's eyes hardened.
"I'll dump the useless brat off here. So long as he doesn't drag anything unnatural into our home, he's your problem."
The priest nodded and mentally checked off the fact the child was abused.
A month later a man came to the church. The child's attention was fully on him the second he stepped through the door.
"So this is the kid?" asked the man.
"You were the first one I called. This kid is powerful John. He can see through more than most," said the priest.
The man bent down to the kid's level. He could see why Father Rick called. The kid had eyes that were too sharp and clear.
"Hello kid. My name's John Constantine."
Cas Shields was an unusual six year old. For one thing he refused to go by the name his aunt had provided. He didn't know who Harry Potter was, but the name felt wrong. So John helped him make a new one. One he would answer to when not in the public school.
Another thing Cas could do that baffled the Father Rick was he had the strange ability to see into people's souls. Some people Cas simply didn't like. He knew the truth about those people because of family and friends coming in for confession, but he didn't know how Cas knew. The fact he seemed perfectly comfortable around John Constantine, yet he couldn't get away fast enough from a known wife beater was very telling.
Today he was letting Cas join in the children's choir.
Five minutes of hearing the child sing and Rick made another mental check that there was something different about him. He had never meet a six-year-old with a voice that strong or pure.
Cas was curled up in a tight ball. He was cuddling his pillow something fierce, and nothing they did would wake him up. It was somewhat amusing to see the not-so-childish boy acting like any kid his age should. Still, if John couldn't get a reaction from the kid within the next five minutes he was going to dump water on his ass.
Cas lifted his head from the pillow to glare at John Constantine. John would have grinned wider, if not for the fact that the kid's eyes had changed colors.
Cas had green eyes. This kid had blue.
"What?" he said irate.
"Just wondering if you were going to grace us with your presence before or after they wiped out the bacon," said John.
Cas blinked twice before he let go of the pillow and raced to the kitchen. John noticed his eyes turned green again. Cas was not a normal kid, but the mere fact he hadn't killed his so called relatives for abuse told him plenty.
He still found it hilarious that Cas was a real carnivore.
John was cursing. This was supposed to be a rescue of a friend's daughter. Astra was possessed and he thought he could remove the thing peacefully. He had rushed in without doing any research.
And now Astra was going to pay the price. If only he had the backing of a Cardinal or higher! They could vanquish this demon without anyone dying or risking their soul!
The demon inside Astra had John pinned so he couldn't exorcise it. He would have only one chance to summon a higher demon to drag this one to hell.
Without warning, the demon started screaming. John's eyes widened in shock as he recognized Cas.
He started choking once the demon was off him, trying to get air into his lungs.
"C...Cas? What are you..."
Cas was speaking a language he didn't understand, but recognized pretty fast. Where the hell did a kid learn Enochian well enough to speak it without mangling the language?!
The demon screamed and a white light came forth from Astra's body. She slumped, unconscious but her soul still very much inside her body. The demon was dead.
"Cas, what are you doing here?" repeated John. He noticed vaguely that his eyes were blue again. John barely caught Cas in his arms. The ten year old was out cold and running a fever.
When he woke up three days later, John was there.
"Cas, what were you doing in that house?"
"House? The last thing I remember was falling out of the apple tree and a sharp feeling of pain..." said Cas confused. It was similar to what Father Rick told him when he brought Cas back to the Church. Cas had been helping retrieve apples from the orchard out back when he hit his head. Without warning his eyes turned blue and he vanished. The priest had been beside himself with worry...right up until John appeared a few hours later with the boy out cold in his arms.
John sighed. Whatever inhabited the boy's body wasn't evil. He had never seen a demon that terrified before, and the way Cas seemed to sense darkness in a human was uncanny. If it weren't for the fact Cas was more human than anyone he had ever met, he would swear the kid was...
His mind stopped cold. Was Cas an angel?
Cas looked up from the dinner he was helping the nice Sister Margaret make for the orphans. There was an owl in the tree with a letter.
Once he finished cutting up the vegetables, he went out to get it. Cas had an affinity to animals. Particularly birds.
He picked up the letter and the owl hooted at him expectantly.
"Wait here," said Cas calmly.
He went into the inner sanctuary where the priest usually stayed.
"Father, an owl delivered this. It's addressed to that name," said Cas.
Father Rick looked up from the finances of the orphanage. This was going to be a particularly difficult year if they couldn't find a way to cut corners without hurting the church.
He didn't miss the way Cas' eyes landed on the budget.
The Father took one look at the letter and winced. This explained quite a bit. Like why that woman was so eager to dump Cas on the Church after school. The boy spent more time here than he did at that house. He would have moved in to the church if he could, but something or someone was keeping him in that home. So he made a compromise with his 'aunt'. He would have sleepovers with the other children for every week and would only spend a night or two at the house.
About the only exception to that fact was the one time a cardinal visited the orphanage and set off the boy. John had been on a case in Ireland, and had only heard about it second hand.
Cas had seen into the man's soul and had reacted violently. Father Rick never did find out what Cas did to cause that reaction in the man, but he later learned the cardinal was suspected of having...relations...with choir boys in the churches he visited.
Father Rick shuddered to think of what that man could have done to Cas, who was by far the most attractive child in the church choir.
"Cas, I want to be very clear on this. Having magic does not make you evil."
Cas blinked, before his deadpan reply had the Father cracking an amused smile.
"Of course magic isn't evil. If it were, then Constantine would be in bigger trouble than he usually gets in to."
"That's right. Magic can't be evil. But it can be used for evil. I don't want to hear about you using this gift to do harm. Are we clear?" said Father Rick.
"What about defensively? To protect myself or others?"
"That is another thing entirely. So long as you always show penance for any life you take, it's acceptable," said Father Rick without hesitation. Then a sly grin crossed his face, "Besides, you're not the only one here with magic."
Cas looked at the Father with shock when he pulled out a ten-inch stick.
"Father, you have magic too?"
"I graduated from the Vatican Academy for Magic. If Hogwarts doesn't work out I could always write a letter of recommendation for you in a few years."
John was the one roped into taking Cas into Diagon. Mostly because he had to get some extra ingredients for his work. He usually hit the smaller areas.
Cas kept close to John, glad that the man had warned him about the famous scar which supposedly the Boy-Who-Lived had. A scar which had disappeared shortly after his second birthday.
"Next!" barked a short creature.
John waited impatiently for the goblin to finish counting jewels. He was one of the few who knew the purpose of the exercise. The goblins did it to annoy the wizards by forcing them to wait.
"Name or key?"
"We want a blood claim first. No point in coming here without making sure he has a vault waiting," said John.
A few minutes later the goblins were giving Cas an odd look.
"Why did you come here without your key?"
"I have a key?" said Cas.
"You have an entire series of vaults. How do you not have a key?"
"I was dumped on my 'aunt' with only a letter and a blanket. I've been living at an orphanage run by a church for the past six years," deadpanned Cas.
"We didn't even know he was magical until that letter came," said John helpfully.
"Where is his magical guardian? They should have been the one to bring him here!"
"Can I register a magical guardian?" asked Cas.
"Three doors down on the left. First we need to re-register the keys. At the moment the only one available to you is the trust vault, at least until you turn fifteen. The most you can withdraw is a thousand galleons, three hundred sickles and one hundred knuts."
John choked. How rich was Cas?
Once Cas had his guardian set to the Father and his secondary as John, he got more than enough to go shopping...and converted five hundred galleons to pounds. John said nothing, as he had an idea of what Cas was planning to do with that money.
In the pet store
Cas was looking around, but nothing really appealed to him. Until he found the cat section. Inside one section was a kitten with a squashed in face and surly look. The moment Cas picked it up, it started purring.
"Find anything good?" asked John. Cas held up his kitten.
"That thing looks like it had a bad encounter with a frying pan," said John.
"I like him."
"Alright, you can get the cat. I need to stop by the Owl Emporium to find something you can use to contact me since they found a way to fry phones at the school."
John spared a look at Cas.
"What are you going to name him?"
"Christo," said Cas.
John snorted. Perfect name for a cat...and it would alert Cas to demons at the same time.
Cas found a pure white owl in the Emporium, which he immediately named Angel. She seemed annoyed by John's cigarettes to the point she kept biting him on the ear when he tried to smoke, to his annoyance and Cas' amusement.
If, later that night Cas dumped more than twice the Church's income for the orphanage in a month, John said nothing. At the very least, the Father wouldn't have to worry about cutting the budget for the foreseeable future...especially since Cas informed the goblins who had really been taking care of him for the past six years.
Apparently there was a stipend set up for him, one that the Dursleys had been getting for years, but never spent a single pound on him for. It would be going to the Church and the 'stolen' funds would be taken with interest from the family as recompense...and then paid to the church orphanage per Cas' instructions. It wasn't like he had any interest in monetary gain.