This will be a Harry Potter and devilman crossover.
To those who haven't read devilman or other works of Kiyoshi Nagai also known by his pen name Go Nagai you should find one of his manga, they are the best I've read and I read a LOT of manga. He often deals with themes not often used having a main character being both hermaphroditic and homosexual and this was in the 70s in his most famous and moral challenging story Devilman. For who is truly more evil? Those who are evil or those who use evil to fight it, or, perhaps it is those who turn on another when evil is made known. This is the very topic of the manga in question. I'm sure I just messed with a lot of world views then. For if the hero is really a villain then what is right and what is wrong?
This takes place after the Apocalypse in the original manga and like a terrible cycle of war, life had began a new. Amon and Lucifer remained on the ruins of earth after Amon's host/possessor Akira died. This takes place both during and before Hogwarts. Be warned, this WILL be bloody dark and I won't censor myself like I have with my other works such as opting for less violence, smut, cursing and death but here I'm gonna go fucking wild.
Book 1, CH 1: the demons who lived
Godric's Hallow was quiet in the cold November night. Not a single thing alive would know of the horrors about to unfold that night. But nothing was said about the dead. Upstairs in the room adjacent to Harry's, James and Lily potter were fast at work. Tomes and tablets scattered across the tables were being scowered for information. A way to win the war and beat the prophecy that they knew hung over the son's head like an executioner's ax. Unnoticed by either potter two figures stood watching them. Both would of ripped out the throat of the other with a spray of blood had it not been for their desire to see the potter heir survive and postpone the second apocalypse for another few hundred years.
The first was a blond eight winged, two on the head, two on the legs, two on the arms and two on the back, hermaphroditic angel known as Lucifer who stood there as naked as they day they were created by god. He wasn't a fan of clothes asides trench cloaks.
The second was a red eyed creature with bat wings on it's head and fur covered legs with blood red wings and fangs. This was Amon the demon hero who once fell victim to bring possessed by a human, that was a major blow to his ego. Almost made it worth Miki fawning over his possessor and by extent him.
Both Angel and Demon were growing impatient after having followed around the couple for two years since the prophecy was first issued. Amon and Lucifer had come to an agreement of sorts. Amon wanted the potter heir due to demon blood on Lily's side. Who it was he didn't know nor cared, he'd have more control than last time and not release it in bursts leading to insanity. Lucifer however wished to help his granddaughter, when she was born, Luna. His daughter Saline was half angel and admittedly was the result of his last attempted merging where as Amon's had made Vlad Tempes the third into Dracula.
"Aha! Lils I found something!" Both looked in through the window, it seems the man finally noticed the tome that Amon left there. It only took them six months. "One with a pure heart can gain protection from a demon!"
"James you moron!" *SMACK* Lily growled, slamming her fist down on her husband's head. "That's ridiculous!"
"It worked for you, didn't it?" James asked, rubbing his head in pain.
"I'm a born Devilman dumbass!" Lily rebutted. *sigh* "you're lucky I love you or I'd of ripped your head off by now."
"He's quarter demon at least lils, it couldn't hurt to try as that blood protection we found only works on pure humans." James explained in a sad tone. Amon huffed in annoyance. If they didn't hurry up he'd just smash the window in and make a deal.
"Fine. But I'm going to transform for this in case it's that fucking turtle again. I bloody hate Jinmen." Lily muttered. Amon frowned at the mention of that turtle. He hated that face stealing moronic prick. Lily had soon stripped out of her clothes proving she was a natural red head and began to transform. Amon nearly choked in shock. She looked like a female version of him accept blue and with tendrils for hair. Son of a bitch! Saylos got lucky.
[Demons old and powerful from hell I... Fuck sake Amon get in here!] Lily snapped, having finally sensed him. Meekly he opened the window and climbed in as Lucifer laughed his female ass off. Lily stood there tapping her clawed foot in annoyance as her husband kept his wand drawn. "Why are you here?"
"I want to help." Amon answered honesty. Big shock. "I don't want to go through another apocalypse and neither does Lucifer, we're each helping someone who can prevent that. If your child dies, we have less than a decade before Armageddon then three years before the apocalypse I don't know about you but hell is boring."
"So, you want to possess our son?" Lily growled, deep and animalistic in nature. James backed up slightly, unwilling to be caught in the crossfire.
"No. I want him to possess me." Amon admitted. "It'll keep me hidden from Sirene and it'll keep him safe."
"Is that all?" Lily asked, her mind already looking for loopholes.
"I'll help protect and train him, no rampages, no feasting on or torturing humans." Amon stated.
"Ink to ink, blood to blood." Lily stated as she gripped the red demon's hand, reciting hell's equivalent to an unbreakable vow that witches and wizards used.
"So mote it be." Amon nodded.
A mystic wind blew through the room. James stepped back, covering his face with his arm as he was forced back several paces. He absolutely hated dealing with devils. Devilmen and DevilMen ladies he could handle but they were mainly trying to help, Voldemort excluded. He had found out awhile ago that Voldemort had taken over a devil somehow despite not being pure of heart.
"I'll keep the kid safe and sane Lily." Amon stated as he let go of her hand. "Be forwarded, while Hell is still relatively sealed up, it is slowly inching forwards again."
"The icecaps." Lily stated grimly. Her only reply was a nod. The polar ice caps actually sealed the gates of hell and the fact the muggles were melting them didn't bode well, 18 or so years and there'd be no stopping the influx.
"I'll be started." Amon stated as he walked further into the house, stopping at the door to dismissively wave Satan aka Lucifer aka Ryo. The disgraced angel scoffed with a blush and flew off headed for his Daughter in St Ottery Catchpole.
The ritual took less than a minute as Amon turned to a fine powdered ash and his spirit was sucked into young harry. The baby cried out for several minutes before calming and gurgling happily and cooing at the shade of Amon attached at his shoulder, much to said devil's embarrassment, luckily only harry would be able to see him.
The ward suddenly bucked as the potter parents eyes sent wide. Voldemort breached the wards.
"Lily, take harry and run! I'll hold him off!" James shouted as he rushed downstairs, spellfire soon followed as lily, now in her human form and clothed picked up harry and rushed to the room where the tomes were kept and banished them before locking themselves into the nursery. It wouldn't do for voldemort to get such tomes. Suddenly the spellfire stopped and footsteps marched up the stairs.
Lily turned to her child with tears in her eyes, unsure if she would make it or not. "Amon, keep him safe or I'll felay you alive, Harry, stay safe and know no matter what, mommy loves you."
The door was blasted open revealing a black cloacked figure with a silver mask and toxic green wings. "Stand aside foolish girl and I'll let you live."
"Fuck you Voldemort!" Lily snarled as she transformed. The dark Lord was stunned before he cackled. "Come to serve me Saylos?"
"Wrong devil you scaly prick!" Lily hissed as she tacked the dark lord through the wall and into the guest room of the cottage. She punched him in the face followed by a knee to the gut.
Voldemort snarled and fired a black burst of energy, cutting through one of Lily's wings before he punched her in the chest. She grabbed his wrist and with a sharp twist, snapped His arm, bone broke the skin as blood splashed the walls. With a swift kick, lily was sent crashing into the desk, shattering it. With a groan she stood up, holding her bleeding side.
"Stand aside moodblood." Voldemort snarled as his eyes flashed crimson.
"Go back to hell." Lily spat. Green energy began to arc at the bat wings on her head. "Technique! [DEVILBEAM]!" The energy cost Voldemort a arm and wing as he coughed up large amounts of black blood. "I wont let you harm my son."
"Avada Kadava!" A sickly green beam struck lily. She fell backwards, alive but severely drained and unable to fight or maintain her form, she could barely even attempt to move to hide her nudity let alone move as voldemort limped out of the room.
"So, this is the child prophesized to defeat me? Oh well. Avada Kadava!" A green flash was seen. Lily gasped softly as tears rolled down her eyes. Her only son, dead. Then a cry. A baby's cry. Her son was alive!