Author's note: I've been thinking about dark!Marauders quite a bit lately and trying to organize my thoughts into fic. This is one attempt to do that. It went in a strange direction towards the end that was not part of the original headcanon but I kind of like where it went.
Disclaimer: Harry Potter belongs to JK Rowling. No money is being made from this project.
For James it starts with the sharp clarity of death. It starts as he stands over his mother's grave, heavily starched black robes rustling softly in the spring breeze, starts as he sees his father crying and feels anger growing inside of him. It starts when he gets a letter in the morning post carried by a raven instead of the family owl, starts when his world collapses around him and he can barely hear his friends' worried questions, starts when everything he's done up until that moment suddenly feels sharp and childish and impossibly hollow.
Remus comes to it with the pain of a bite fresh in his mind. He learns through books and crumbling scrolls, through whispers in dark corridors and forbidden trips to Knockturn Alley. He learns by experimentation and exhaustive research, by flattering some and bribing others. He learns by will, but he starts by bite, by the sudden agonizing feel of teeth on young skin, by feel of foreign saliva and the smell of rotten flesh. He starts when a predator chases his prey and wins, exacting his price in skin and muscle and dooming his young victim to a life forever in shadows, needing only a small push to deny light entirely.
Peter falls into it without quite meaning to. He follows where his friends go and by the time he realizes where the path leads it's too late to turn back. He does as he's told and keeps his mouth shut and his ears open. He stumbles onto rituals never meant to be seen and hears spells never meant to be spoken. A rat goes where it likes and his senses are keen. He falls backwards into a world he never wanted to find and learns to swim before it drowns him.
Sirius grew up with it and rejected it with every fiber of his being. He burns with the strength of his hatred and his disgust, burns so harshly it hurts even his own eyes though he forces himself not to blink. He ran as far as possible, ran so far from everything he knew that when he stopped be barely even recognized himself. He hisses at anything that reminds him of where he'd come from and inches farther away with every step. inches so far away that in time he finds himself right where he'd started.
James leads the group as he always does, leads with the easy assurance of a man who is accustomed to being followed. He does not try to convince or coerce, merely states his plans and gives his friends the choice. He looks at them with empty hazel eyes and when he raises a hand his fingers tremble. Sirius screams and nearly throws things while James watches in silence. In the end the very possibility of refusal keeps Sirius from walking away; if he chooses to follow James to Hell it will be a choice made freely. Remus objects only on points of logistics and practicality while Peter says nothing at all.
Their pranks get darker, more vicious, more daring. Where once they aimed for humiliation they now strive for pain, for punishment. They target those who will take the enemy's brand when they leave school and show no mercy. The teachers watch them more closely than ever but James turns his blank gaze in their direction and they keep their mouths shut. James wields his charisma like a weapon while Remus sharpens his intellect to a vicious point and Sirius reaches deep into wells of knowledge he'd sworn never to touch. Peter says nothing but his spells are the cruelest of them all. For the first time James looks at him with approval.
Their other friends pull away, unsettled by the changes. Lily Evans refuses to have anything to do with them and James says nothing at her departure. Always insular, the four become nearly exclusive, intertwined to the point where it becomes hard to tell where one ends and another begins. The Slytherins who once sneered in their direction look at them with curiosity turned to respect turned to fear. The Marauders feel that fear and revel in it, feeding it with pranks turned cruel and threats hissed in the dark. From the High Table the Headmaster peers down with concern.
After finishing school they vanish, slipping into the dark as though they had been born into it. James' house turns into a home base, a fortress protected by blood magics with no place guarding a bastion of the light. They have an air of hunger to them now, a sharpness they never had before. James' eyes stay empty even as his hair grows wilder and his gestures less indolent. Remus relaxes, his daily stride becoming a graceful prowl better suited to the wolf he carries inside of him, while Peter moves impossibly lightly. Sirius blazes as brightly as ever, a constant presence at James' side, the fire to his friend's ice. Death Eaters fear their names where once they laughed at them and members of the Order tremble in fear and disgust.
They don't bother with disguises or charms. They walk proudly through the streets, heads held high, expressions dangerous. Peter has been taught the pureblood mask of supremacy while James has acquired a predator's easy gait. Remus' fingernails are long and hard; Sirius has let his hair grow out. Magic floats around them in an intoxicating cloud, drawing followers and attracting challengers. They dismiss both and barely hear the whispers that follow wherever they go. James once collected admirers like sweets but he has no use for them now and it is left to Sirius to draw information from them, milking them expertly and discarding them when they run dry. The people of Britain call them the Dark Princes and fear them nearly as much as the Dark Lord himself.
The Dark Lord vanishes before they can get to him, defeated by an infant seemingly forever. The four have studied enough of darkness to know better than that but neither can they find him no matter how many methods they try. His followers turn to them as a last resort, ignoring the way the four targeted Death Eaters above all others in order to cling to power wherever they can find it. The four massacre those who come to them and feel no remorse. They melt into the shadows, vanishing into the wild as though they never existed. The blood wards on Potter manor stay untouched even as nature reclaims the structure and magic seals its secrets away. The house waits for masters who will not return, a silent marker of what might have been.
Young Neville Longbottom, Boy-Who-Lived, saved by a mother's love and a father's sacrifice, hears their story as a warning. He listens with wide eyes and swears on everything he has that he will not go down that path. Dumbledore, with more wrinkles in his face and more weariness in his eyes, watches him worriedly and does not know whether to be relieved or concerned when the Sorting Hat places him in Gryffindor without a single moment of hesitation.
In the forests around Potter manor the local villagers hear inhuman howls in the night and close their doors more tightly as deep in the woods four young men with once bright futures scream their anger for all to hear.