"But know this, Harry. Do not waste your pieces. Only turn trusted people that can be useful to you."
"Useful? I don't care about that, they're my-"
"Then they will die alongside you. Voldemort is a weakling compared to members of the Church that will hunt them for breathing or others that will use you for their power. Whether you had remained human or became a devil, any world that relies on magic has the same rule as governance."
Harry immediately knew what he was talking about. Even though Fudge, and later Thickness, became the Minister of Magic, it was people like Voldemort or Dumbledore that made the real calls.
Power rules over everything. He had always been lazy in schooling, but schooling didn't do anything. Writing essays, brewing potions, or learning runes didn't make you a powerful wizard. Harry never cared much about the academics of Hogwarts anyway. He wasn't a Ravenclaw for a reason.
"You are welcome to invite as many humans or non-humans into your peerage as you wish. With your new status, if you wish to join devil society immediately, it's best you complete your peerage as fast as you can for the next rating game. There are just too many forces out there to live a peaceful life without relying on a faction. But you needn't rush to complete it either. Enjoy your life for once. Stay in the wizarding world or go to Japan to meet others your age like I told you. You now have plenty of time to spare, Harry."
Harry leaned back against the wall, breathing in and out. At first, he wanted to give up on life and join his parents. The only thing that kept him back was the fact that his friends were in danger. He only calmed down when Cadaer told him that time passed differently while they were speaking in his mind like this. The Hallows had plenty of time to charge the energy required to turn him into a devil and have this much-needed conversation.
But a world where he would never be the Boy-Who-Lived? There's a whole other world out there, where he wouldn't be forced to learn what people told him, where he could be his own master, free from pressure and manipulation. He didn't need to learn a new language or change anything. All he needed was a loyal peerage and power. It all sounded like a dream. Yet, it was a goal. It was something to do and look forward to.
He had once told McGonagall that he wanted to become an Auror, but he never expected to live to 20. Now? His childhood was hard, but 17 years was nothing to a devil. After he got over the shock, his perspective had changed. He died, but he isn't dead. He wanted to live and see the world outside Surrey. Outside Hogwarts. Outside a world where his face was on wanted posters and he was forced to live in a tent.
"A few more things and we can wrap this up," Cadaer grinned. He looked almost youthful. The rapid changes from the image of a battle-hardened, old soul to a mischevious man in his prime had been confusing at first, but it was comforting now.
"As I've said, a Beleth has the power over death. In combat, this usually translates to withering and burning your enemies life-force until they turn to dust."
Harry was nowhere near used to how casual the old devil was with humans and death in general, but he tried to roll with the punches.
"Out of combat, however, it's much more... esoteric."
He paused before a grey mist erupted from his hand and shot out, forming semi-solid chain links around the piece of Voldemort's soul.
"Power over death is what you make it, but with dead things and souls, we have a connection. Intrinsic feelings that you'll get used to and learn to understand. This shard of your old foe has been connected to you your entire life, so it's no trouble to give you the knowledge it had gained over its lifetime. Personally, I think you should witness the depravity and cruelty for your personal growth, but something tells me you don't want that."
Harry glared at his new grandfatherly figure with a deadpanned face.
"Eh," Cadaer shrugged before he shot out more grey energy at the fetus and ripped 3/4ths of it out before crushing it to dust and dragging the remains in front of Harry.
"In a few decades, you'll regret it, but you're your own man, after all," he hummed.
"I don't want anything from him. The knowledge alone is probably as depraved and dark as his experiences."
"And I don't care. If you want to die then that's your prerogative. I will not let it happen because you're an idiot."
"But-"
The devil cut him off, "My familiar will deliver your Evil Pieces to you in a few days after I get everything set up. Speak to Gremory or Sitri in Kuoh when you wish to know more. You're of age for another year of school, so at least you can enjoy yourself and meet some peers. I trust you'll be able to kill your archenemy with ease thanks to his knowledge up to his defeat by your mother's protections. Also, with those Pheonix tears bonding to your mortal blood, it should give you partial regeneration. Parts of your organs and fingers should flame back to normal with little issue."
"But-"
"The Phenex sacrificed hundreds of Pheonixes during the Great War to get their regenerative immortality. Humans aren't the only race to drive a species towards extinction. They'll flip out if they knew your method, so we'll claim your mother was a distant Phenex cousin and you were my heir hiding with wizards for fun since you're eccentric."
"Wha-"
Harry couldn't get another word in before the Lord Beleth whipped the remains of Voldemort's broken soul directly into Harry's body.
He didn't know it yet, but a mere human would never be able to absorb so much knowledge without dying. He had turned into a devil the second that Cadaer appeared. His change left his overall character untouched, but it's in the very nature of devils to be impulsive and true to themselves.
The human Harry would've never run towards a fight unless he was forced to. He was completely selfless because of his low self-esteem and crushed ego that always had him following orders like Hermione and left him looking up to others from the ground, despite his celebrity status.
The devil Harry was still the same heroic and trusting person that lived up to this point. The only difference is his new instincts. Instincts that told him how he should be proud of his accomplishments. Who cares if he was forced into situations? He came out alive because he was better.
He was proud of himself and he felt a new sense of wrath towards Voldemort.
He wasn't a Fallen Angel though. One of them would have become obsessed with the two sins and reveled in them.
He was a devil. While Angels exist for virtue and the Fallen exist for sin, devils exist for domination. Some dominate for lust, some for wealth or influence, some wish nothing more than to dominate in combat, while others can even live to dominate for benevolent reasons. A devil is true to their nature as long as they bow to no one whose power does not completely trump their own.
It's why there were four Maous, not kings. On paper, they're mere figureheads. In reality, they're figureheads that others look up to with respect, jealousy, or as a goal. It works for devil society.
In the reincarnated devil's case, he felt his own instincts pushing him towards the bastard that ruined his entire life.
Tom Marvello Riddle came for him first.
And Harry Potter would crush the paltry wizard underneath his foot.
I'll try to use canon Harry Potter while I can, but expect major DxD changes as Harry builds his own peerage.
So far, I plan on a knight, bishop, rook, and 3 pawns from HP canon. 3 of them are very minor characters.
Only Luna would leave Britain (but she's hard to write).
Hermione wouldn't become a devil on principal, Ginny is a house elf name, Ron is a flake, Tonks is dead, Fleur is married, and Nev wants to make his ancestors proud. You will see HP characters in devil contracts though.