It sure looked like Tanjirou beheaded Rui last week with his Hinokami Kagura Breathing, but alas; in the moment before Tanjirou’s strike hit Rui severed his own head with his threads, and soon reattaches it. He’s mad as hell, and Tanjirou is totally spent, but it’s okay that he can’t lift his hand, because Tomioka Giyuu arrives to finish Rui off with ease, using an eleventh form of Water Breathing.
From there we cut to the lone surviving member of the spider family, the elder sister, and we learn about how Rui built his family. Turns out all his family members were really weak demons with whom he shared his power—which also gave them the same spidery aesthetic. He used their fear to draw them in, an punished and even killed those who didn’t shape up.
The present-day sister once had an older sister who tired of Rui’s pointless charade, and vowed to run away, telling only her sister so she could join her. However, our present-day sister betrayed the other by leading her straight to Rui, who tortured her and strung her up to be burned away by the morning sun.
Back when Tanjirou saw Rui cutting his “sister’s” face, we didn’t know what was going on, but Sister’s face reverted out of fear once Mother and Brother were killed. It’s her first screw-up, but it isn’t her last. That honor goes to when she encases Murata in one of her yarn balls, which fills with digestive fluid that will liquify his clothes and eventually, him.
Murata is saved by one Kochou Shinobu, fresh off of curing Zenitsu. When Sister insists Rui made her kill the scant five people she’s killed, Shinobu has proof she’s lying, as she saw over a dozen of the yarn balls in which Murata is stuck, and estimates the Sister has eaten up to eighty humans. Shinobu agrees to be her “friend”, but only after she’s faced proper punishment for the people she’s killed.
Hayami Saori voices Shinobu like she would any sweet, friendly, kindhearted young woman, only the words she says are anything but sweet. I’d even say Shinobu relishes the chance to show off her unique Insect Breathing ability, whereas Giyuu is much more stoic and businesslike. You can hardly blame her; both her graceful dance-like movements, her delicate blade, and clouds of butterflies make for a hell of a show.
When the Sister realizes she hasn’t been beheaded, that Shinobu lacks the strength to do so, she believes she still has a chance to gain the upper hand. But she’s wrong, because while Shinobu didn’t behead her, she did poison her with Wisteria, resulting in a slower and arguably more gruesome and painful death. She doesn’t burn to ash, either; she’s simply dead, and Shinobu can’t be bothered to do anything but leave her corpse to rot.
With that, we jump back to Rui’s final moments, when he looks back to how he tried to regain memories of his humanity by creating a pretend family. But by now it’s a bit late to engender any sympathy for the guy, nor his treacherous sister who led her sister to a horrible death.
Unlike Nezuko, who has yet to even accidentally kill a human, these demons have long since forfeited any chance of mercy by preying on untold numbers of humans. They were living on borrowed time, and that ran out when they ended up on the wrong end of Giyuu and Shinobu’s blades.