While Akira was fighting Nakime elsewhere in the Infinity Castle, Erika was locked in a far more dangerous battle of her own, facing Upper Moon Rank One, Kokushibo.
Sanemi and Muichiro were engaging him directly at close range, their blades clashing again and again, while Genya and Erika provided long-distance support from the rear. Yet even with all four of them working together, it wasn't enough. They weren't gaining ground.
If anything, Kokushibo was toying with them.
The realization made Erika's chest tighten. As she dodged another distant crescent slash, she reached out telepathically.
"Selena… are the reinforcements coming?"
Selena's voice answered calmly. "The Stone Hashira is nearby. He should arrive soon."
Erika let out a small breath of relief. "Okay… then what is Akira doing right now?"
There was a brief pause.
Selena replied flatly. "He's crying about how you didn't tell him before you fought. Or invite him to the fight."
Erika froze mid-step.
Only then did it hit her.
She had promised she would inform him. She had promised she wouldn't rush into danger alone.
And she forgot.
"Oh no… I forgot", Erika thought back urgently. "Is he mad? What should I do?"
"It's just my brother", Selena replied. "He'll probably forget about it if you leave him alone."
"No," Erika immediately shot back. "I broke my promise. And I know he was worried back then too. What if he stays mad at me?"
Selena hummed for a moment, then said lightly, "Why not give him a lap pillow and ruffle his hair like before? I'm sure he'll be happy."
Erika's face instantly burned red.
"B-But that's…"
She didn't get to finish.
A crescent-shaped moon blade tore through the air toward her.
Erika barely twisted aside in time, the blade grazing past where her head had been a heartbeat earlier.
"Erika-Chan", Selena snapped. "You are in the middle of a battle. Maybe concentrate on that."
Erika inhaled sharply, her heart pounding.
She realized just how stupid she had been, letting her thoughts wander in the middle of a life-or-death fight. All she had wanted to know was whether reinforcements were coming.
But the moment Akira had been mentioned, her focus had slipped.
For some reason… whenever it was about him, she always got distracted.
Erika clenched her fists, steadying herself, and forced her attention back to the battlefield.
This was not the time to think about those things; she was in the middle of a fight.
Erika let out a slow breath and forced herself to focus.
Right now, Sanemi and Kokushibo had just exchanged a violent clash. Steel rang through the vast chamber as their blades met. Sanemi managed to hold his ground for a moment, but Kokushibo's strength overwhelmed him, forcing him back several steps.
Even so, Sanemi remained steady on his feet.
Kokushibo regarded him calmly and spoke, his voice strangely distant.
"Long ago… it was the Sengoku era…"
His tone slowed, almost reflective.
"…and this is the way I… yes… how the Wind Pillar and I… both improved our swordsmanship skills…"
For a brief moment, it looked as if he was reminiscing, his six eyes unfocused, drifting through memories long buried.
Then, without warning, he attacked.
"Moon Breathing, Sixth Form: Perpetual Night — Lonely Moon, Incessant."
Kokushibo unleashed a storm.
A multitude of curved slashes erupted several meters in front of him, releasing a wild barrage of crescent moon blades that shredded the air itself. The attack expanded outward, slicing through everything in its path.
There were too many.
The massive pillars surrounding them were cut down instantly, stone collapsing as if made of paper. Erika, Muichiro, and Genya, positioned farther back, managed to block or evade the incoming blades just in time.
But Sanemi wasn't as lucky.
A deep, brutal slash tore across his chest.
Blood sprayed into the air as he staggered back, the wound carving straight through him.
Kokushibo looked down at Sanemi, his expression unreadable.
"Hm… you held out for quite some time up to this point," he said calmly.
"But if you move now… your organs will spill out."
As the words left his mouth, something felt off.
His pulse…It spiked.
"…What?"
Before he could think further, Sanemi threw his head back and burst into laughter.
"FUFUFUFUFUFUHAHAHAHAH!"
Blood spilled freely from his chest, soaking his uniform, but he didn't slow down.
"Cat to silverine," Sanemi snarled.
"Demon to Marechi blood."
He charged forward again.
Kokushibo staggered, just a little.
Sanemi grinned savagely.
"Demons get drunk once they get a whiff of my blood," he said.
"And this blood's the rarest of the Marechi blood, a once-in-a-lifetime shit."
He raised his blade.
"Now have a feast."
Despite his torn body, Sanemi attacked again.
"Wind Breathing, Sixth Form: Black Wind Mountain Mist."
He rotated his body in a powerful uppercut motion, unleashing a violent tornado of slashes that ripped through the space between them.
At the same time, Muichiro rushed in.
"Mist Breathing, Fourth Form: Shifting Flow Slash."
He took a low stance and dashed forward, his blade flashing as he slashed toward Kokushibo in a smooth, flowing strike.
Erika and Genya joined in as well, attacking from a distance, ribbons snapping and bullets firing, to buy even a few seconds for Sanemi to recover.
But Kokushibo was relentless.
He pushed through their attacks without pause.
Then.
CLANG.
A massive flail wrapped in chains smashed into Kokushibo's side, forcing him back and finally buying Sanemi some breathing room.
Kokushibo turned his gaze toward the newcomer.
"…One appears after the other," he murmured.
From the shadows stepped an enormous figure.
The Stone Hashira, Himejima Gyōmei, had finally arrived.
"…The Demon Slayer Corps will never disappear," Gyōmei said, his voice deep and unwavering. "…Until we eradicate… all demons from this world."
Relief and renewed resolve spread through the battlefield at the arrival of such a powerful reinforcement.
Gyōmei turned his head slightly.
"Shinazugawa," he said. "Stitch up your stomach wound right now. We'll take him on while you do that."
Sanemi clicked his tongue.
"Sure. Sorry about that."
He immediately pulled out the blue potion Akira had given him and drank it, his wounds beginning to knit together as he stepped back.
Gyōmei then faced Kokushibo.
Holding his axe in one hand, he spun the flail rapidly with the other before hurling it forward.
Kokushibo dodged.
He attempted to counter.
Moon Breathing—
But before he could complete the technique, Gyōmei hurled his axe straight at him.
Kokushibo narrowly avoided it.
Gyōmei pressed the attack without pause.
"Stone Breathing, Second Form: Upper Smash."
The chain flail flew again.
Kokushibo dodged, but this time, the chain wrapped around his throat.
He tried to sever it with his katana.
It wouldn't cut.
His eyes widened slightly as he ducked just before the chain tightened.
Gyōmei didn't let him escape.
Again and again, he attacked relentlessly.
However, while Kokushibo's wounds regenerated instantly, Gyōmei's injuries did not heal at the same pace.
Kokushibo spoke calmly.
"Everything that breaks, I regenerate right away. Your attacks are useless, pitiful human."
Gyōmei clenched his teeth.
He knew it.
If he wanted a real chance to win, he would need to activate his Demon Slayer Mark.
But he had been saving it for Muzan.
"But…If I fail here…"
Before he could finish the thought.
Sanemi and Muichiro, now recovered, rushed back into the fight.
Genya resumed his cover fire.
Erika's ribbons lashed out as well, binding and striking wherever they could.
Gyōmei joined them again, deciding that, for now, he didn't need the mark yet.
Their relentless assault finally tore through Kokushibo's purple kimono.
His body was wounded.
But every injury healed in an instant.
Kokushibo looked down at the torn fabric.
"You tore my kimono," he said evenly.
"You have my praise."
Then his katana began to change.
It grew longer.
Branches of blades sprouted from it unnaturally.
"But you couldn't kill anything with this."
His gaze sharpened.
"Once I deal with two of you… The rest will be easy."
And then, Kokushibo attacked.
"Moon Breathing, Seventh Form: Mirror of Misfortune — Moonlit."
Using his altered katana, Kokushibo unleashed a frontal crescent-shaped slash. Multiple crescent moon blades followed the strike, interlaced with countless straight slashes that expanded outward through the ground itself.
Everyone barely managed to dodge.
But the attack did not stop.
Without pause, Kokushibo continued.
"Moon Breathing, Eighth Form: Moon-Dragon Ringtail."
A massive, wide, long-ranged curved slash erupted from his blade, dozens of crescent moon blades lining its trajectory as they tore through the air.
Then.
"Moon Breathing, Ninth Form: Waning Moonswaths."
Multiple downward curved slashes followed in rapid succession, all packed closely together, each leaving behind layers of crescent moon blades that shredded everything in their path.
Still, it didn't end.
"Moon Breathing, Tenth Form: Drilling Slashes — Moon Through Bamboo Leaves."
Three rotating, circular saw-like slashes formed, crescent moon blades spinning violently along their paths as they tore forward.
The attacks kept coming.
Everyone did their best to dodge.
But no one could evade all of them.
Sanemi, Muichiro, and Gyōmei were riddled with wounds.
Blood splattered across the broken pillars and shattered floor.
Erika and Genya fared better only because they were fighting from a distance.
Even so, the barrage never ceased.
The moon blades continued to rain down relentlessly.
Erika felt her chest tighten.
"At this rate… we're going to lose."
She knew it.
She had to do something.
But there was nothing she could do.
She couldn't use her Noble Art; there was no earth here.
She couldn't rely on her Blood Demon Arts; it was useless against this kind of overwhelming, omnidirectional assault.
Even if she turned invisible, Kokushibo would still see her through the Transparent World.
And the others alone weren't enough to turn the tide.
For the first time since the battle began.
Erika didn't know what to do.
Unable to find an answer on her own, Erika sent a desperate thought through the link.
"Selena… what should I do?"
There was a brief pause.
Then Selena replied, "Well… why not learn and use Kokushibo's Blood Demon Art? Those crescent moon blades. I'm sure you could win with that."
Erika snapped back immediately. "Do you think I'm Akira? I can't just learn something like that on the spot!"
Selena fell silent again, thinking.
"Then…If you can't do it yourself… why not have someone else do it?"
Erika blinked. "What do you mean?"
"You may not be able to use a Blood Demon Art suddenly," Selena said carefully, "but there's someone here who can, right?"
Erika hesitated.
Then realization struck.
Her head snapped to the side.
"Genya."
While dodging Kokushibo's relentless attacks, Erika sprinted toward him. As she moved, she scooped up a broken fragment of Kokushibo's flesh sword from the ground.
She reached Genya and thrust it into his hands.
"Eat it," she said urgently. "We don't have time."
Genya froze in shock.
But he understood immediately.
His Nichirin bullets hadn't done any damage to Kokushibo. He hadn't known how to be useful. This…this was his chance.
After only a moment's hesitation, he clenched his teeth and swallowed the fragment.
At a distance, the front line was somehow still holding on.
Kokushibo raised his blade, preparing to continue his assault.
When Nichirin bullets suddenly reached him.
He dodged effortlessly.
Only for the bullets to curve midair and strike him instead.
Kokushibo's eyes widened.
"What is this…?"
He turned toward the source.
Genya stood there, but he no longer looked human.
His eyes had turned black. Fangs protruded from his mouth. Even his gun had transformed, resembling Kokushibo's own katana, flesh, eyes, and all.
Suddenly, a tree burst from Kokushibo's body, rooting him firmly in place.
Erika's breath caught.
"This is our chance!"
She moved instantly.
"Blood Breathing, First Form: Crimson Thorn Prison."
Her ribbons warped around Kokushibo, wrapping him tightly, locking him in place as gravity crushed inward.
"Wait…Erika-chan!" Selena cried.
But Erika didn't stop.
The pressure intensified.
Seeing Kokushibo restrained, the Hashira reacted at once.
"This is it!" Gyōmei roared. "Cut off his head!"
All three Hashira charged in simultaneously, attacking Kokushibo with everything they had.
But Kokushibo refused to fall.
With a roar, he unleashed all his power.
Blades erupted from his body. Crescent moon blades exploded outward in every direction.
All of them struck Erika's Device Églantine, which was wrapped around his body.
The ribbons trembled.
Then.
They shattered.
The moment they broke, Erika's mind went blank.
Her strength vanished instantly.
Her legs gave out, and she collapsed to her knees, unable to stand.
As the ribbons containing the attack shattered, a massive shockwave exploded outward, blasting everyone away.
Erika's body felt unbearably heavy.
She tried to push herself up, but her strength was gone.
Her limbs wouldn't respond.
"So… this is it…"
Selena's voice burst into her mind, frantic and panicked.
"Erika-chan! Are you alright?!"
Erika tried to answer.
But she couldn't.
Every bit of energy in her body had vanished in an instant.
Her thoughts drifted, and suddenly she remembered Akira's words from before.
"When a Device breaks, the user usually loses consciousness immediately."
The only reason she was still awake was because she was part vampire.
Her vision blurred.
Then, she felt it.
A presence above her.
Erika's eyes widened.
She looked up.
And saw Kokushibo's blade descending toward her, merciless and absolute.
She had no strength left.
No power.
No way to move.
No way to fight back.
She couldn't even raise her arms.
"I… can't…"
Death closed in.
Erika slowly shut her eyes.
And in that final moment, only one thought crossed her mind.
"Akira…save me…"
Then.
Something happened.
A deep crimson light engulfed her body.
Kokushibo's sword struck.
But instead of flesh, it hit metal.
The impact released a violent shockwave, blasting Kokushibo backward.
Erika gasped and opened her eyes.
Her vision cleared.
She looked down at herself.
A familiar Jet-black armor covered her body.
Her breath caught.
"…Eclipse…?"
Yes.
She was wearing Akira's Device—Eclipse.