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Published:
2025-09-03
Updated:
2025-09-07
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6,714
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3/?
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The Shape of Your Darkness

Summary:

Marauders x Arcane AU

Riddle’s near ten-year hold of the Undercity has not been in vain. When an important development happens in his favor, the final stages of his plan to conquer Piltover and rule as the one, true leader of Zaun are underway.

Sirius Black is the closest to a right-hand Riddle would allow, deadly in his craft and ruthless to keep order. He’s thrilled at the prospect of Piltover falling in repentance for its brutalities against the Undercity. When Riddle sends away his brother to spy on Piltover’s “Golden Boy”, James Potter, Sirius is left making an unforeseen connection with a tall, lanky boy from the Lanes who plants seeds of doubt in his loyalty to the cause.

Twenty-year-old and Zaunite, Regulus Black, works loyally under Riddle’s command as an ingenious chemist of the Sledgehammers — but he has doubts. His mission was supposed to finally convince him that Riddle’s rule would be glorious, but the famous, golden-hearted inventor with glasses seems to be having the opposite effect. An effect he can't afford unless he's willing to make the greatest of sacrifices - losing his brother.

Notes:

Author's Note: I use the "Undercity" and "Zaun" interchangeably to describe the Undercity! Also, I tagged it, but I promise I do NOT use AI to write my fics - I just love a good Em Dash ;)

Also, I have no set timeline for publishing chapters, but know that I plan for this fic to be a long one, and I'm constantly writing!

Chapter 1: Responsiblities

Summary:

Death Eaters Sirius and Regulus Black have worked loyally under Riddle's command of the Undercity for years. Sirius is Riddle's highest-ranking Death Eater, constantly finding himself cleaning up messes he did not create. Regulus is the lead chemist who transformed Zaun's chemtech into a thriving production. Although his mind is valued more in the laboratory, Riddle has other plans for him.

Notes:

(See the end of the chapter for notes.)

Chapter Text

Sirius

Patience had been the theme of Sirius’s day.

One of their secret warehouses was attacked early in the morning, reduced to dust before sunrise. Luckily, it wasn’t a crucial one — mainly building materials — but still. Inconvenient. Inconvenience meant meetings. Endless talk of oversight, risks, and blame.

Patience.

Talk of Sirius’s capabilities to solve such an issue.

Patience.

As if it were his fault. Bulstrode was the one who supplied the manpower for such security tasks – or any task for that matter. Sirius created the moving schedules in and out of the warehouse, perfectly curated to avoid drawing attention to the small, ruined building on the outskirts of Zaun. If the building had burned, it was because Bulstrode’s men failed. Yet it was Sirius who carried the weight, who cleaned the messes. Always Sirius. 

That’s why the heir of Black now crouched on the rooftops of Zaun, waiting for the Piltover supply airship to fly overhead to replace the crates of steel lost from this morning’s incident. 

Patience. Which was now running thin.

“It’s behind schedule,” Barty muttered, squinting at his cracked watch. 

“Could be weather.” Sirius let the irritation edge his words. “We’ll wait another hour.” It was a half-assed excuse; the air was still calm and warm enough to collect sweat under his black cloak. 

Crouch pocketed his watch and nodded. The Rosier twins echoed Barty’s compliance, shifting into more comfortable stances. 

The three figures before him mimicked shadows hidden underneath black cloaks identical to Sirius’. Evan Rosier was the only one still sporting the gold-painted skull mask; if the heat affected him, his body language revealed nothing. 

Sirius knew Barty Crouch the longest, nearly a decade now. Even so, Barty was much closer to Regulus, Sirius’s younger brother. Besides himself, the boy was the only one who could make Reg smile before his brother turned into a moody git. Their history gave Barty unrelenting confidence to be complacent on assignments, but it was tolerated only because he matched his arrogance with lethalness. 

Plus, the twins offered a nice balance, both level-headed and quiet for the most part, but just as deadly. Quiet was an overstatement for Evan Rosier; Sirius had yet to hear the boy speak once. Perhaps his tongue was cut out as a child, or maybe he had nothing to say. Evan tossed a throwing knife between his hands – the objects always left his grip with precision that even Sirius couldn’t replicate. 

Pandora, by contrast, was sharp, her wit flashing with the same intensity that burned in her eyes during combat. The twins were yet another connection made through Regulus, more victims of Piltover’s brutalities and the Undercity’s lack of protection at the time. She was the one to offer their services to Sirius in return for income a few years prior, and they’ve earned the title of usefulness.

Pandora whispered something to her brother before pulling away with a grin that revealed the slight gap in her smile. Meeting Sirius’s eyes, she winked and tucked a loose lock of snow white hair back into her black hood. 

A low hum reverberated through the rooftops, snapping Sirius’ attention. Through the clouds, the hulking silhouette of an airship pushed through the air above them. He put a hand up to signal to his team, who had all shifted into ready stances. As the back of the airship disappeared back into the clouds, Sirius unstrapped the driftplate from his back. He slid his mask down and launched himself into the night, steel slicing the wind. The metallic clanks behind him confirmed that the others followed in pursuit. 

The driftplates had been spoils from an attack a few months back, where he and his accompanying men were ambushed on a routine patrol. The element of surprise gave the Firelights the upper hand, but the onslaught was poorly planned — and Sirius was deadlier. He was reprimanded that day for losing a few of Bulstrode’s lackeys, but it was a win in his mind. Almost a dozen Firelights dead, and five driftplates Sirius and his team had been abusing since. The flying metal boards gave them an advantage in ground combat and other raids like the one they were on now. 

The cover of night made it easy to fly across the underside of the airship and sneak onto the deck. Gold and blue danced off the propeller that whipped the wind in a low purr.  The nice thing about Piltover’s cargo ships was the lack of security, but after this raid, it would be bound to tighten for future shipments. Black crept underdeck into the cargo hold, his team following silently behind. Sirius pulled his mask off once they deemed the coast to be clear,

“Remember, only attach the parachutes to the crates with steel. Evan.” Rosier’s pale lashes caught a glint of light as he nodded and vanished toward the stern. The rest of them made quick work of securing crates with plate-sized metal disks that sprouted claws into the heavy wood.

Parachutes would trigger with the push of the purple button in its center. Having a remote to trigger them all at once would’ve been ideal, but the techwright designer, Yaxley, couldn’t make one on such short notice. All the crates were dragged into rows away from the stern. Twelve in total. 

When Evan rejoined them, he slid the setonator into Sirius’s hand. “Right then,” Sirius checked his watch. They should be right above the river now. “On my count.”

They each lined up with a row of crates. Sounds of groaning wood suggested that the wind was stronger up here. They’d have to be quick.

“Three, two, one.”

Sirius pressed the detonator, ducking behind the crates to avoid the wood debris flying through the air following the explosion. The airship shook violently, the sound of the blast echoing through the sky and getting lost in the clouds. Sirius hastily regained his footing,

“Now!”

The quartet shoved the crates through the gaping hole now in the stern of the airship and into the night sky. Crouch and the Rosiers leapt through right after the cargo. Yells and footsteps of the crew echoed overhead, but it was too late. Sirius lingered just long enough to drop a pair of goggles and an old handgun — both in his possession courtesy of the Firelights — before leaping from the wreck.

Stronger winds threatened his balance on the driftplate, but Sirius leaned into it, narrowing his eyes against the rush. The dim city lights below revealed his crew sailing through the air, triggering the parachutes on each crate. He counted eight parachutes already airborne, four left. Sirius leaned forward, hovering through the air with precise grace — the remaining parachutes were triggered in a matter of seconds.

The cargo hit the river with hefty splashes, bobbing with the ripples of the murky water. Luckily, all of them seemed to have remained intact. Neon green lights emerged from the city canals, trailing behind small boats right on cue. He watched the silhouettes of his crew load the steel, the hulls dipping under the weight. Earlier, he convinced Bulstrode to lend him a few men to drive the boats back to a new warehouse under the condition that Sirius covered the difficult part. It wasn’t too hard a persuasion considering Sirius’ towering rank and a threatening reminder that the loss of the materials was originally Bulstrode’s fault anyway.

Crouch and the twins appeared next to him to watch as the boats sailed back into the canals of Zaun. Moonlight glittered in the river, highlighting a few debris from the airship following the blast. Above, smoke trailed from the wounded airship. Below, the river rippled with stolen wealth. All that remained was to wait and see if the brutes could successfully transport the cargo without fucking up.

Patience

⫘⫘⫘⫘⫘⫘

Regulus

Regulus hissed as bright purple liquid spewed from the beaker and nearly singed his arm. He etched a note to slowly mix in the silt for future reference. The concoction was meant to fortify lighter metals to provide better durability while adding little to no weight. It would be used in the forging process right after heat treatment. 

“Black,” a nasally voice interrupted, “the batch for today’s meeting is ready.”

“Good. Bottle them then leave, Severus.”

Severus’s straight hair fell over his brow as he nodded and turned back to his station. Regulus was initially annoyed when Severus was assigned to work for him, but he’d proven his usefulness to carry out the tasks that would make Regulus die of boredom. The assistant was a gift from Riddle for Black’s accomplishments in the laboratory, but Regulus couldn’t help but be insulted. 

Within the past three years in Riddle’s service, Regulus made more breakthroughs in chemtech than the rest of the inner ring had in the seven years prior. Various bomb compounds, the gas lamps that illuminated the streets a greenish-yellow (he didn’t particularly care for aesthetics), and a handful of shimmer variants didn’t even compromise a fifth of his contributions. Providing a spare to share his lab with was incredibly inconvenient, but one could not refuse Riddle’s generosity.

The laboratory doors swung open, followed by a familiar gait that drew a groan from Regulus. Rows of glowing purple vials mounted on the walls rattled in their cases.

“You’re going to destroy my lab,” he said, not bothering to look up. Scattered footsteps suggested Severus made a quick exit. Hopefully not before doing what Regulus asked of him, or he would flog the greasy assistant personally.

“I don’t give a shit.” Sirius’s voice carried its usual arrogance. He strode over and leaned against the desk. Regulus could feel his movements being watched. “How do you see anything you’re doing in here?” 

“With my eyes.”

Sirius huffed. “What’s he got you working on?”

“Strength variant,” Regulus muttered, annoyed.

“Shimmer does that plenty already, no?”

Regulus rolled his eyes, “It’s not a shimmer variant. And not for people, for materials. Seeing as you keep fucking around on those driftplates, they might as well be indestructible.”

Sirius snorted.

“Were you able to intercept the shipment?” Regulus asked, finally looking up at his brother, who nodded in response. The low purple and green hue emitting from the ridiculous amount of vials in the room clung to the high points of Sirius’ face. Black curls fell right at his shoulders and got lost in his matching black leather jacket. Regulus always thought his brother looked like death incarnate. Not the kind that was loud and barbaric, but the kind that slithers and weaves, devouring in silence. Even so, Regulus would never be afraid of him. While Sirius’ dark presence might reverberate coldly in others, Regulus wore it as a blanket that promised safety. 

Although he despised being confined to the dingy, murky lab all day, Regulus never envied Sirius’ position. He was decent enough at combat himself, but he wouldn’t be in the inner ring of Death Eaters if it weren’t for his brains. A handful of the lower-ranking idiots credited his status to being the second son of Walburga Black — the business mogul who controlled the largest district of Zaun and oversaw chemtech distributions — an accusation that made him grind his teeth. 

Sirius’ talents were noticed by Riddle early and purposely redirected in the benefit of his operations, while Regulus’ were underestimated. It’s true, he could have resided in being coddled at Black manor after completing his primary studies, but Regulus got bored easily. Being underestimated proved to be an advantage in clawing his way upward. Everything must be earned in Zaun; no one was an exception.

Sirius’ gaze hardened, “Riddle’s going to be there today. At the meeting.”

“Must be important then.”  Riddle’s presence at meetings was rare, especially since most of them were stupid anyway and didn’t call for all Death Eaters to be present. If he was expected, it usually meant something very promising or severe had happened. Sirius would be the only one to know the answer ahead of time. If his brother was nervous in the slightest, he didn’t show it. Perhaps good news, then. 

Sirius rolled his neck and pushed off the desk, handgun clicking in the holster as he did, “C’mon, let’s walk there together. Don’t forget the tray.”

Regulus internally cursed at himself for dismissing Severus so quickly. The idiot could have carried the tray to the meeting room for him. 

The vials of shimmer clinked as they walked through the Ashlock Foundry. The seven-storied building was once a factory of some sort, but it now served as the main headquarters for Riddle's operations. The building was located deep in Zaun, in one of the poorer areas near the Lanes. Its location was a mediocre representation of the Undercity, but a necessary cover. The interior gave anyone who entered whiplash from how starkly it contrasted the nearby area. Hideous botanical wallpapers combined with the patterned rugs made the hallways an eyesore. Rich, mahogany tables and plush chairs were neatly littered throughout the rooms of the foundry; Regulus’ lab was no exception. The only parts of the building that reflected taste were the windows — Even Regulus could see the beauty of the stained glass windows that glittered in different shades of emerald green. The meeting chamber was especially magnificent, with glass covering the ceiling, designed in a way that looked like overgrown vines creeping up the walls. But there was a smell. It was dense and a bit smoky, and it clung to his clothes. Even the luxuries inside Ashlock Foundry could not fully repel the foulness of the lower Zaun districts. 

Regulus and Sirius were the last to arrive — save for Riddle. There were no greetings, only low mutters between members. He placed the tray down, feeling the greedy eyes of the other Death Eaters that loomed over him. The vials were plucked instantly, needles plunging into their skin with grotesque satisfaction. Regulus could feel the disgust coiled in his gut, but he’d long stopped letting it show. He and his brother were smart enough to know never to mess with shimmer themselves, tempting as it was at times.

Sirius took his seat to the right of the head, but Regulus would take his across the table and three chairs down next to Bulstrode, the brute who provided manpower for operations. He controlled The Lanes, the second largest district of Zaun, but his power still lagged behind his mother’s and the Black family. The bitterness often tainted his interactions at meetings because the seating arrangement was a constant reminder of his lower Death Eater status in comparison.

Riddle valued order and recognition of his power; none of his luxuries came free, and everything was a reminder that being in his service was the highest honor. Even the seating arrangement at meetings had a silver lining. Selwyn sat to Regulus’ left, farthest from Riddle. The two remaining small districts fell under his eye. As the underground liaison, Selwyn had a network of spies running through the Undercity and a handful in Piltover as well, to Regulus’ knowledge. Obnoxiously tapping his metal leg under the table across from Regulus sat Crouch Sr., the smuggling overseer and Barty’s father. He was a stocky man whose eyes constantly fluttered about, as if he physically couldn’t spare his focus on a single thing at once. Barty detested his father for being cowardly, and Regulus wasn’t one to argue with the fact. It was a miracle he was a Death Eater at all, although Riddle probably liked having a more susceptible mind close to his disposal. 

Yaxley, the techwright designer credited for all of the chemtech designs, sat in the next higher seat adjacent to Crouch. And then there was Sirius — The highest ranking Death Eater and the closest thing to a right hand Riddle would allow. 

Beyond Bulstrode to the head’s immediate left sat Walburga Black, overseer of Alcove District and head of the Sludgerunners, the illegal enterprise that specialized in manufacturing and distributing chemtech. His mother. 

The doors swung open dramatically, and everyone turned to acknowledge Riddle’s entrance. He was a tall, slender man who walked with a frightening calm. No hair clung to his bald, pale head. Instead, the attention was always drawn to his tar black eyes with dark yellow irises. Regulus would be lying if he said he wasn’t somewhat impressed by the man’s ambition. Riddle’s unattractive looks were evidence of the obstacles he had to overcome to climb to his position as the leader of Zaun — the unofficial name for the Undercity he plotted to sever from Piltover. Sirius and Regulus would make a game of guessing what specific doings graced him with his appearance. Regulus was certain Riddle was the victim of an explosion at some point, which would explain the missing nose and mechanical left hand.

“Reports,” Was all he said to kick off the meeting. 

Updates circled the table, but only some topics were of interest — a new secret trade route was finally done with construction, the Firelights’ attacks have gone down significantly, suggesting a strong decrease in numbers, and four successful smuggling operations since the last meeting. When it came to Regulus, he mentioned that the strength variant was still in progress, but was still ahead of schedule. Still, Yaxley gave a disproving click,

“Why not consult Vaska?” His voice was husky through the gold gas mask concealing his nose and mouth, “I have weapons designed that need the enhanced armour now.

Regulus resisted the urge to roll his eyes and instead gritted his teeth at the insult. Vaska, the crazy bitch of a scientist who first produced the shimmer formula, sends Regulus batches to experiment with per Riddle’s orders. The original formula of the drug would never be revealed to anyone, but that doesn’t stop Regulus from trying to figure it out anyway, purely out of spite. She was technically still under the command of Riddle but worked independently, on darker projects he would want to keep secret, Regulus assumed. Before he could respond, Riddle interrupted, his voice slick like ice,

“Vaska is under commission for a special task. Regulus’ servant will have the results by next month.” 

Servant? Severus? The boy was a tool compared to Regulus, like most people. Why the fuck would Riddle replace him for such a task? Rage flickered, but Regulus kept his tone flat, “My Lord?”

He ignored Regulus and brought his snake-like gaze to the entire table, “I bring good news today. I have secured the loyalty of someone very important. Someone with direct access to Piltover’s council.” 

Hushed murmurs scattered.

“A council member? Of Piltover?” Bulstrode asked, brows furrowed.

Riddle gave a truly hideous smile. “Precisely.”

Excitement surged through the Death Eaters. Regulus caught Sirius’s expression — unreadable and unsurprised. Riddle stood from his seat, silence following his movements. “Our new friend will act as my eyes and ears on the council, and will be a key part in eliminating Noxus as a threat in arms for the time being.”

Noxus was Piltover’s most valuable ally, mainly because of its powerful and ruthless military. As a child, his tutor explained that Noxians valued strength above all else — even their government was built on meritocracy. Riddle would only see Noxus as a threat if they were to turn on Piltover, and there would really only be one reason why they would need to impose themselves —

“With this movement, my plan to conquer Piltover will finally draw to its final stages,” he adopted a menacing tone that promised blood, “My nation of Zaun will be triumphant. Topsiders will pay for their insolence and mediocrity. And of course, you, my Death Eaters, will drown in the riches of the Upper City’s finest. ”

Thuggish hollering sounded throughout the room. Bulstrode slapped his hands against the table, and Sirius had a wicked grin on his face. This was it. It was bound to happen at some point. Regulus knew Riddle wouldn’t aim so low as just to sever Zaun from Piltover; he needed all of it. Suddenly, the past year came into clear light. Yaxley had been pushed to design all sorts of maniacal weapons and artillery. Walburga was tasked with expanding the Sledgehammers tenfold. Even Regulus had been pushed to spend more hours in the lab the past year. A hiss sounded from the head of the table,

“Even with Noxus’ artillery neutralized,” Riddle braced his gloved fingers on the table, “Topside still remains ahead in technological advancements. My source on the council claims there have been no great advancements in their weaponry, but I want a closer eye kept.”

Riddle snapped his neck to look at Regulus, 

“Regulus, you will be sent to Piltover Academy under the sponsorship of our new council friend. With his resources, you will get close enough to the ‘Golden Boy’ to report back his technological advancements.” A mocking sneer coated the mention of the renowned inventor.

If his jaw wasn’t clenched, it would be resting on his lap. It’s true that Piltover steadily drew farther ahead in terms of technology with the invention of the Hexgates — Large towers that teleport airships across the continent — courtesy of James “Golden Boy” Potter. Their creation transformed Piltover into a global shipping lane, which granted them continuous prosperity and power that only grew in the past two years they’ve been in use. Of course, the Undercity saw little to no of its benefits. Even with the growing supply of chemtech, if the two cities were to go to war at this moment, Zaun would perish. 

His palms threatened to draw blood from how hard he dug his nails into them. James Potter. Piltover’s darling. With no direct support from Zaun. How long would he be away? He refused to look at Sirius, whose gaze weighed heavily upon him. 

Fucking control yourself, Regulus.

“You will depart in a week with details to follow.” Riddle rolled his next and turned. Sirius stood to follow suit. “Dismissed.”

Notes:

This is my first fic! I really hope u guys enjoy it bc I have some really good ideas planned. So far, I don't have a consistent timeline for publishing chapters, but know that I write whenever I can! Also, if you think I should start adding TWs in the beginning notes, let me know.