Two days after Gyokko's death was announced, Zenitsu decided to visit the Butterfly Mansion to see Nezuko. Lady Amane had given the three of them a week long break as a reward for killing an Upper Moon. Thanks to this, they were all free.
'Hehehe!'
He couldn't help but grin wildly. While he decided to go play with Nezuko, Tanjiro and Inouske had traveled to the Swordsmith Village to get new swords.
'Nezuko-chan comes first!'
He arrived at night, unwilling to spend the day alone while Nezuko slept.
"Ah! You!"
Aoi yelped when she saw him suddenly leap into the compound. She immediately wanted to scold him, but when she remembered his rank, her expression shifted.
"You... You shouldn't sneak into other people's apartment by night."
She pointed at him, still indignant.
"Hehe! My bad." He laughed awkwardly, scratching his hair. "Is Nezuko-chan awake?"
Aoi's eyes narrowed. So that was his original goal for coming here.
With bit of mischievousness, she smirked.
"Yes. But she's currently washing at the sprin—"
"Hiiiii?!"
Zenitsu's eyes bulged as he clasped his hands, teeth chattering.
'N-n-n-n-n-nezuko-chan, b-b-b-bathing?!'
The thought sent him reeling.
Aoi smiled victoriously, seeing his current expression. There was an even greater surprise on the way.
"Welcome home."
A soft, childish voice interrupted the Thunder Hashira's thoughts as he turned to the voice like a robot—his movements mechanical.
There, stood Nezuko in her regular attire. The bamboo around her mouth hung loose, dangling from her neck.
She was looking at him with a warm smile that made Zenitsu feel like jelly.
'D-did Nezuko-chan just speak?'
"Welcome home."
Zenitsu felt an electric shock pass through him. Nezuko had just welcomed him, not with her usual hum, but with words.
"Uwahhh!"
In a blur, he arrived before her, holding her hands between his.
"You must have been waiting for me to come right? I knew it, Nezuko-chan, we were meant to be together. You even spoke because of me. Ahhh! This is so amazing. I feel unlimited."
He continued ranting until her childlike voice interrupted him again.
"Welcome home, Aoi-san."
Zenitsu froze. All his bodily functions seized, then went into restrained overdrive.
Veins bulged on his head as he turned to look at Aoi. The girl had an innocent expression on her face as if she had no idea what was going on. Her heartbeat said otherwise though.
Thump!
Just then, he sensed another heartbeat nearby, familiar yet unfamiliar. Familiar because it sounded like the heart of someone he knew and sounded like a demon's, then unfamiliar because he couldn't place a finger on who or what it was.
'Who?'
"Long time no see, Zenitsu."
Zenitsu's expression soured as he finally confirmed the person before him. A young demon in black kimono and a blue haori stepped into view, a sword by his side. His eyes had turned black, with only his irises green.
And those eyes, they held the Kanji; Three.
"Kaigaku."
The demon mentioned grinned in maniacal joy, enjoying the grim atmosphere that hung above the mansion.
"Feeling fear already? Master's favourite..."
Zenitsu remained quiet, his expression grim. Aoi was quite surprised by how easily his expression changed. Even Nezuko was looking at him with a peculiar look.
"You..." He growled, hand tightening on the hilt of his sword. "Do you know what you've done?!"
He lashed out, expression twisted in anger.
"You were supposed to inherit Thunder Breathing! But because of you! Because you turned into a demon! Master had to personally kill himself in the most agonizing way. I'll never forgive you!"
Silence blanketed the mansion as both stared at each other, eyes glaring with rage and focus.
Kaigaku's laughter shattered the silence.
"Hahahahaha! That old man died? Haha—hahahaha! He got what he deserves! I was supposed to inherit the Thunder Breathing, true. But then he decided that I'll have to share it with you, a wimp? Hah!"
"So what? You learned the Second Form to the Sixth Form but couldn't learn the First Form. I did! Master had great expectations for both of us! And you! You—"
"Shut up!"
Kaigaku roared, a flash of lightning erupted from him. A section of the butterfly mansion walls crumble.
"Respect? That senile fool favored you—you, the coward who faints at blood! He deserved to die choking on his own regrets!"
Zenitsu froze, then his expression eased.
"I see." He closed his eyes, then drew his sword. "Then I can kill you peacefully."
Turning to the side, he spoke to Aoi.
"Get the others and hide. I'll try to get him away from the mansion while you contact the other Hashiras."
Nodding, the girl took Nezuko and ran into the mansion.
"Well," The Upper Moon Three grinned. "Ready to die now, Zenitsu?"
"I won't die."
The Thunder Hashira whispered, then moved. Taking his stance, a hot breath escaped his lips.
"Thunder Breathing: First Form—Thunderclap and Flash."
Gold lightning spilled from his form, cackling with intensity.
He moved in a straight line and struck at Kaigaku.
"Pathetic."
Clang!
Kaigaku blocked with a casual swing, then erupted with blue lightning.
"Demon Thunder Breathing: Second Form—Rice Spirit."
From above, arcs of lightning struck Zenitsu, rolling around him like a prison or cage. The lightning crawled across his body, electrocuting him repeatedly.
"Ugh.."
He groaned in pain, struggling against the attack. His entire body felt numb and his vision blurred, but he refused to fall.
"I will... not..."
He bore through the lightning, slowly regaining mobility.
"..die before I kill you!!!"
"Thunder Breathing: First Form—Thunderclap and Flash: Sixfold."
He suddenly appeared in six different places, then fused into one behind Kaigaku.
The demon frowned, then felt blood trickle from his arms, legs and a small nick on his cheeks.
"Heh~ Still pathetic." He sneered, masking his surprise. Zenitsu, that weak idiot, was able to injure him, the Great Upper Moon Three?! The thought ignited a furious flame in his heart.
"You're weak! You're weak! You're weak! You're sooo WEAK!!!"
He clashed with the Hashira, their swords grinding against each other in a spark of steel. He was insanely fast, moving in and out of Zenitsu's vision like a glitch.
The young Hashira could barely keep up, his arms trembling. His eyes slowly relaxed, and he muttered.
"Thunder Breathing: First Form—Thunderclap and Flash: God Speed."
The night sky split open, and everything was consumed in a golden flash.