I don't want to say too much and give everything away before you even read it, but I'll set things straight right now so that it makes actual sense. (As much sense as my stuff ever makes, anyway.)
Voldemort does not exist.In this Death note Harry Potter mixture of sorts, Tom Marvolo Riddle AKA Lord Voldemort has never and will never exist. Someone else takes that role, without actually fulfilling the role. It's... kind of weird and hard to explain. You have to read it to understand, but I must insist that you take everything you know about Harry Potter canon and throw it out the window because it doesn't exactly apply here very well. This is more of a strange twisted Potter world.
There's something oddly soothing about waving a wand - Swish and flick, Mr. Keehl, and don't forget to pronounce it correctly - and repeating the same phrase over and over to Mello, and he doesn't quite understand why but it is and that's enough for him.
Swish, flick - mustn't forget the flick Mr. Keehl, you know, that's what drives the spell - pronounce. Repeat.
Nate never forgets to flick.
Mello would love to hit him with an Unforgivable Curse, but the disappointed look on Dumbledore's face wouldn't be worth the trouble. Sometimes he doesn't care what Dumbledore would think, and those are the times when he takes his wand and his Charms books and hides in the Astronomy tower to wave his wand and practice.
He hears the whispering in the halls about the boy with only a letter for a name, hears how he's such a genius and only the strange boy Nate River is close to his level of talent.
Mello's fellow Slytherins give him looks of disgust, the perpetually angry blond boy with the filthy Muggle parents who abandoned him when they discovered their strange son had 'magic'; Dumbledore had promised to tell no one that his parents had abandoned their son to the care of the young Professor upon discovering his was not normal, but somehow the secret had gotten out as secrets were wont to do.
He didn't care about his parents so much. He'd never liked them anyway, and he'd stopped referring to himself as Mihael Keehl once he'd come up with a better name for himself.
Nothing stupid like 'Lord Mello', or 'Dark Lord', or anything. Just 'Mello'.
It takes a bit of effort to maneuver himself into the good graces of his Housemates -they still dislike the filthy Mudblood among them, but Mello can be persuasive when he wants something and even Dumbledore was a fool for his charming wit when Mello was on the hunt -, but eventually Mello has a small group wrapped around his finger and following him around Hogwarts like puppies. Of course, most of the Fifth year's small group are Fourth and Third year Slytherins and a single Second year, so it was easy to make them do as he pleased.
Most of their time is spent tormenting younger students (mainly of the Gryffindor variety), and practicing the Dark Arts on small animals. Mello excels at his Defense Against the Dark Arts classes, but excels more so at the Arts themselves.
This is a fact he kept away from Dumbledore, who'd become his unofficial guardian. The summers he spends in the tiny London orphanage are for his own good, Dumbledore promises, because it will keep him grounded in the Muggle world and help him to remember his 'roots'. Mello has the sneaking suspicion that Dumbledore is not interested in reminding him of his 'roots' so much as he is interested in trying to keep Mello from becoming too wrapped up in the power trip he receives from lording over his group.
Nate has no such group, and Mello loves the thrill of knowing he has something Nate lacks, natural leadership skills.
The L boy isn't much better. He is a popular topic of discussion - "Did you hear? That L kid got another Outstanding in Potions and it only took him five minutes to finish his potion!" -, but students avoid him like he's some sort of freak, and he isn't known for being friendly anyhow.
Mello loathes him as much as he loathes Nate.
It doesn't take long after leaving Hogwarts for Mello's emotions to best him, and soon he is known across England for his use of the Unforgivable Curses; most notably the Cruciatus Curse, in solving disputes.
Dumbledore wonders where he went wrong with the blond, but Mello promises that Dumbledore did nothing wrong before the green bolt drains the life from his Professor's eyes.
I don't know exactly what is up with this. I'm thinking of adding more chapters. I guess you could call this a prologue-trailer of sorts?
Well, either way, the world of Harry Potter will be completely different with Dumbledore dead, if I do end up continuing this. I'll leave this up to you guys to decide if I should continue with this, I guess.