Episode 40

Transformation (変貌) is the 40th episode of the Kimetsu no Yaiba TV anime series, and the 14th episode of its second season.

Synopsis
As Nezuko rushes to aid Tanjiro during a critical moment, her appearance begins to gradually take on a demon form as she sustains Daki’s attacks. Nezuko is able to overpower Daki as her demon form accelerates; however, she turns her attention to innocent bystanders injured during the battle and charges towards them. While Tanjiro attempts to restrain Nezuko, Daki draws near...

Plot
After healing her leg, torso and left arm, Nezuko emerges from the rubble. She grows in size, attains vine-like patterns in her body and a horn on the right side of her head. She charges towards Daki, who instinctively cuts her leg and arm again. However, she heals instantly and slams her foot hard into the Upper Rank's back.

Reeling in pain from the impact, Daki struggles to understand how she healed so quickly, realizing too late that Nezuko's regeneration now exceeds her own as the new demon stomps her off left arm. Hearing her screams, Nezuko smiles sadistically at her opponent. As Tanjiro lies unconscious from his clash with Daki, a vision of a tearful Takeo tries to get his brother to awaken, saying their sister won't be the same anymore, causing the Demon Slayer to quickly regain consciousness. Nezuko breaks off Daki's other arm, causing her to scream in pain again, only for Nezuko to mercilessly continue her assault. Daki uses her obi to stave off Nezuko, slicing and decapitating her attacker. The Upper Rank backs away and sends her obi towards the young demon, who unexpectedly stops the attack while still not whole, using her congealed blood to maintain form and fight. Suddenly, Nezuko's splattered blood ignites, setting Daki alight. Remembering a horrific incident in her childhood, she screams in agony as she is roasted alive. Seeing this, Nezuko rejoins the pieces of herself and stares in delight at Daki's torment.

Nezuko leaps into the air as her Blood Demon Art finishes, slamming her leg down onto the frightened Daki. She forces her down into the earth repeatedly, before kicking her down the road and chasing the hurtling demon with her new speed. Unable to do anything, Daki is forced to watch as Nezuko violently kicks her again, driving the helpless demon through several house with great force. As she approaches her, Nezuko suddenly smells the blood of an injured bystander. As she tries to fight back the urge, her teeth chitter as she loses control and walks towards the wounded girl. She succumbs and moves forward to kill her when Tanjiro arrives in time, using his scabbard as a muzzle.

Nezuko tries to fight her brother off, scratching him as he begs her to fight the urge and apologizes for forcing her into the battle. Sensing that Nezuko's blood is strewn around the area, Tanjiro realizes just how injured she is and asks her desperately to stop and sleep to heal. She gets down on all fours and leaps into an upper storey of the house with Tanjiro on her back. Still fighting the pain from his lack of breath using Hinokami Kagura, Tanjiro begs her to go to sleep. A horrifically burned Daki enters the scene, her healing still hampered by Nezuko's fire. With innocent lives nearby, Tanjiro is forced to choose between a rampant Nezuko and facing Daki. When she uses her obi to attack him, Tengen emerges into the house and cuts them before they can hit. The Sound Hashira calmly asks if this is truly Nezuko, now "a flashy demon". When Tanjiro asks him for help, Tengen can only growl angrily at her new state, in spite of showing her resistance to humans in front of Kagaya and the Hashira. Daki asks if Tengen is here to fight her, but he only tells her to shut up and claims she isn't an Upper Rank. As she angrily retorts this, her head suddenly falls off, sliced in the neck by Tengen earlier.

As Inosuke and Zenitsu stand alone on the rooftops, the former rants at losing sight of Tengen. Using his hearing, Zenitsu detects a distant explosion and runs towards it, as Inosuke follows suit. As Tanjiro stares in disbelief seeing Daki truly beheaded, Tengen reminds him the battle isn't over and instructs him to control Nezuko. As he half-heartedly suggest singing her a lullaby, she kicks into the floor and flings herself and Tanjiro outside. Realizing she isn't listening to him, he asks his mother for help and thinks of an idea. Still reining her in, Tanjiro sings a lullaby sung by their mother and holds her hand as she struggles. Nezuko suddenly sees herself not in Tanjiro's arms but her mother's. Kie sings the lullaby and the demon remembers a time when her family journeyed into the mountains.

As they walk up the mountain, Nezuko asks about the song she sang, innocently wondering if her brother's eyes were red because Kie had eaten red berries while pregnant, causing Kie to smile brightly at her fun question. Nezuko begins to cry and eventually controls herself, becoming smaller and falling asleep; exhausted, Tanjiro tells their mother she's finally asleep. Preparing to walk away, Tengen hears Daki calling to him, saying he will pay for beheading him. Uninterested, Tengen tells her she is of no use to him and to die "an unflashy death". Daki tries to claim she is truly an Upper Rank demon, only for the Sound Hashira to condescendingly ask why she is holding her head. She tries to claim again she is Upper Six but Tengen doesn't listen. Insulted, Daki begins to cry and whines she is Upper Six and that she is amazing. Unable to believe he's witnessing a demon throw a tantrum, Tengen suddenly realizes she isn't disintegrating despite being beheaded. As she rants and pounds the floor in anger, Daki suddenly calls out for her "brother".

At that moment, something emerges from the stump of her neck and grows out her body. Tengen speeds forwards to decapitate the new threat but the demons vanish before his swords could reach. When Tengen turns around, he stares in horror at the sight of two demons; Daki's brother calms her crying and heals her burns instantly. Tengen attacks the new demon, blocking his blow but sustaining a cut to the head. As they face each other, the demon compliments his face, body and musculature. When he claims Tengen receives much attention from women, he claws himself in envy, telling him to die a horrendous death. Daki tells her brother there are more demon slayers, asking he kill Nezuko who burned her and claiming she tried to do her best but "everyone got in my way and bullied me". He calls hurting Daki unforgivable and prepares to exact his revenge, while threatening that Tengen will spin around as he dies, "Because my name is Gyutaro".

Gyutaro hurls his sickles towards Tengen, causing a large explosion and decimating the house. As Tanjiro watches from below, he sees the flying sickles boomerang back inside and suddenly smells Tengen's blood. To his relief, Inosuke and Zenitsu emerge to assist. Taking Nezuko away, he instructs the two to enter the house and help the Hashira. Having stopped the attack with his blades, Gyutaro sees this and takes time to compliments Tengen as "a good-looking guy" and noting the frightened bystanders next to him must believe he is their saviour. At this, Tengen agrees and adds that he even has three wives. Infuriated, Gyutaro angrily calls this unforgivable and infuses his blood into his sickles, before using Blood Demon Art: Flying Blood Sickles to slice him to pieces. Analyzing the razor thin blades, Tengen deploys bombs to blow the demons away and destroy the floor. The Hashira lands to the bottom and urges the civilians inside to flee. Seeing this, Gyutaro causes his blood sickles to bend and chase after him, forcing Tengen to fend them off as they lock onto him.

As he fights, Tengen realizes just how powerful the siblings are and wonders if perhaps they would both die if Gyutaro is beheaded instead. Using his hearing, he echolocates the house and is relieved to see everyone has fled. He hurls more bombs into the air and uses his swords to direct and activate them, destroying the house and knocking Inosuke and Zenitsu to the ground. After the blast, Tengen jumps up to survey the damage and sees a ball of obi land unharmed. As he remarks this mission won't be easy, Daki unfurls her obi while Gyutaro states, "The thing about us, is that we're two people in one!".