u/Greedy-Deer9726

**Infinity Castle Arc: Kutsuki Sabiha’s Stand**

The Infinity Castle’s shifting corridors had already claimed many lives by the time Sabiha landed. The wooden floors twisted beneath her feet like living serpents, disorienting her sense of direction. She moved with the patient, measured steps of someone who understood decay—rushing would only hasten her own end. Her rust-orange haori fluttered behind her, the fabric stained with the dust of earlier battles.

A familiar scent of wisteria and blood cut through the air. She rounded a collapsing hallway and nearly collided with **Tsuyuri Kanao**, who was sprinting forward with her Nichirin blade drawn, eyes wide with rare, burning determination.

“Kanao!” Sabiha called, voice steady despite the chaos. Kanao glanced back, surprised, but nodded sharply. No words were wasted. The two young women fell into step—Kanao’s precise, flower-like movements complementing Sabiha’s slower, grinding attrition style. They raced toward the overwhelming aura of Upper Rank Two.

They burst into Doma’s frozen domain just as the horror unfolded.

Shinobu Kocho’s broken body was disappearing into the demon’s gaping maw, her final defiant smile still etched on her face as the poison coursed through him. Doma laughed brightly, blood and wisteria toxin dripping from his lips. “Ahhh~ What a shame! She tasted so bitter-sweet!”

Kanao’s breathing hitched. “Shinobu-sama…!”

Sabiha’s grip tightened on her Nichirin katana. The sight of the Insect Hashira—someone who had shown her quiet kindness during training—being consumed ignited something cold and furious in her chest. “We end him here,” she said quietly. “Before he digests her poison… or her.”

The battle erupted.

Kanao moved first, her **Flower Breathing** a whirlwind of graceful, lethal strikes. Sabiha followed with heavier, deliberate forms, her blade trailing reddish-brown particles that clung to Doma’s regenerating flesh like acid.

**Third Form: Flaking Barrage** — Sabiha spun into a storm of slashes, scattering rust flakes across Doma’s arms and torso. The particles hissed and smoked as they ate into his icy skin, slowing his regeneration just enough for Kanao to land deeper cuts.

Doma giggled, unfazed. “Ooh, a new toy! Your rust is so pretty~” He unleashed a barrage of ice lotuses and fan strikes, forcing them to dodge across the frozen platforms.

They fought in sync: Kanao’s speed creating openings, Sabiha’s corrosion wearing him down. Sabiha switched to **Fourth Form: Galvanic Decay**, spinning her blade in a defensive vortex of oxidizing particles that corroded Doma’s incoming ice constructs mid-air, turning them brittle and shattered.

But Doma was Upper Rank Two for a reason.

In a blindingly fast motion, he closed the distance. His fan, edged with razor-sharp ice, whipped across Sabiha’s face. She barely turned her head in time—**the fan sliced deep across her right eye**.

Pain exploded. Blood poured down her cheek, hot and stinging. Her vision in that eye went dark, replaced by a burning haze. She staggered, clutching her face as rust-colored blood mixed with her tears.

“Sabiha-san!” Kanao cried, covering for her with a desperate **Flower Breathing**.

Doma tilted his head, smiling serenely. “Does it hurt? Good. Let’s make it match your pretty rust theme~”

Something inside Sabiha snapped—not rage, but the cold, inevitable truth of decay. *Everything breaks down. Even demons.* Her breathing deepened. The pain in her ruined eye pulsed in time with her heartbeat.

Then, the **Demon Slayer Mark** awakened.

Jagged branches of rust under her left eye.

**Sixth Form: Structural Collapse** — Empowered by the Mark, Sabiha lunged. Her blade moved with terrifying weight, superheated by concentrated oxidation. The slash connected with Doma’s side, carving through his flesh as if his immortal body were crumbling rusted iron. Chunks of his torso flaked away, regeneration visibly struggling against the mark-enhanced corrosion.

Kanao, seeing the opening, pressed the attack with renewed ferocity, her eyes filled with tears and resolve.

Together they drove Doma back across the frozen hall, buying precious time. Sabiha’s voice was low and steady even through the agony in her eye: “You consume everything… but everything returns to rust in the end.”

Doma’s smile faltered for the first time, his body smoking from the combined poison and rust corrosion. He laughed again, but it sounded strained. “How interesting…! But I’m still not done playing~”

The fight raged on, the two Slayers standing together against the demon—Sabiha’s newly awakened Mark burning bright, her right eye a bloody ruin, yet her will unbreaking.

1_First Form: Surface Corrosion

A quick horizontal slash that coats a wide area of the demon's body in rust particles, weakening regeneration in that zone for several seconds and making follow-up cuts more effective.

2_Second Form: Pitting Strike

Thrusting stab that creates small explosive bursts of rust along the entry point, digging deep "pits" into muscle and bone, disrupting movement.

3_Third Form: Flaking Barrage

Rapid multi-directional slashes that scatter rust flakes like a storm of razor-sharp metal dust. Great for crowd control against weaker demons.

4_Fourth Form: Galvanic Decay

A defensive spinning maneuver that creates a swirling barrier of oxidizing particles. Any demon limb or attack that passes through it begins corroding instantly, slowing and weakening the limb.

5_Fifth Form: Red Rust Cascade

Overhead downward slash that releases a waterfall-like cascade of heavy rust particles, weighing down and corroding everything in a narrow forward cone — excellent for pinning fast demons.

6_Sixth Form: Structural Collapse

A single, extremely powerful vertical cut aimed at the neck. The blade "rusts through" defenses by superheating the edge with friction and oxidation, making it feel like the demon's neck is crumbling apart even before the blade fully severs it.

7_Seventh Form: Eternal Patina (Personal ultimate form)

The user channels everything into one long, sweeping circular slash that envelops themselves and the target in a dome of thick rust mist. Inside, regeneration is almost completely halted while the user's blade gains extreme cutting power from concentrated oxidation heat. It ends with a final thrust that aims to sever the head while the demon is too decayed to resist. (High stamina cost — usually a last resort.)

She's literally Rust Hashira and the most flirtatious girl in corps that others gave her Flirt Hashira (though her flirts are mostly for fun not something serious).

I'm starting a massive collaboration since summer is close. I'll choose someone then they should draw their oc(s) here and post till the next day. And they should choose the next person like me. So if you are in I choose mine in the next post.

Sabitha was born into the Kutsuki family, a long line of rural blacksmiths in a remote mountain village near the base of Mount Natagumo (close enough to tie into canon events subtly, but isolated). For generations, the Kutsuki forged not just farm tools and household blades, but also Nichirin swords and repair work for passing Demon Slayers—quietly supporting the Corps without seeking glory. Their forge was legendary among lower-rank Slayers for producing durable, reliable blades that held their edge even after brutal battles. The family motto, whispered in the heat of the anvil: "Even the strongest steel yields to time... but it fights until the last flake falls." Her father, Kutsuki Tetsuro (a stern but kind master smith), taught her the art from childhood. Sabitha showed unusual talent—not just in hammering metal, but in understanding its "soul": how impurities created strength, how heat and quenching mimicked life's trials, and how rust was not failure but nature's honest verdict. She helped temper blades, learning Total Concentration Breathing basics instinctively through the rhythmic bellows work and controlled breathing needed to avoid burns. The tragedy struck on a cold autumn night when Sabitha was 14 (21 in the main story time). A powerful demon—drawn by rumors of "blades that never dull"—attacked the village forge. It slaughtered her entire family: her parents, older brother (who was training to join the Corps), and younger sister. The demon mocked them, saying their precious metal would "rust in demon blood anyway." Sabitha survived only because she was fetching water from the stream; she returned to find the forge in flames, bodies strewn like broken tools, and the demon feasting. In blind rage and grief, she grabbed her father's half-finished Nichirin blade (still warm from the forge, its color shifting to a dull rust-red from the heat and blood). Using the breathing techniques she'd learned unconsciously while working the bellows, she fought back—not with speed or power, but by wearing the demon down. She slashed shallow cuts that left rust-like corrosion, slowing its regeneration until dawn broke and the sun finished it. The blade, exposed to her desperate breathing and the demon's ichor, permanently took on that reddish patina—becoming her personal Nichirin sword. Orphaned and alone, Sabiha wandered until she encountered a retired Demon Slayer who recognized her breathing pattern as a crude offshoot of Stone Breathing (with erratic Wind-like flurries from her frantic dodges). He brought her to the Corps for training. There, under guidance from Stone Hashira-affiliated mentors (perhaps a connection to Gyomei or a lower pillar), she refined her style into Breath of Rust—turning the family's legacy of forging unbreakable steel into a weapon of inevitable decay.