Spider Family

The Spider Family (蜘蛛鬼家族) was a clan of demons on Mount Natagumo with spider-like traits and abilities, formed and led by Lower Rank Five, Rui, of the Twelve Kizuki.

Overview
The Spider Family was a group of five demons: Father, Mother, Daughter, Son, and Rui, who portrayed himself as the youngest brother of the family but was actually the true leader and most powerful of the group. However, the clan at one point had nine total members, previously having two additional sisters and brothers. They were most likely killed by Rui for failing to meet his expectations in their roles as his family members, leaving only the last four present and himself.

The Spider Family was an oddity, as demons typically don't form groups. Although calling themselves a "family," the clan was highly dysfunctional at best and downright violent and sadistic at worst within their own kin, with torturous levels of physical and psychological abuse being commonly practiced. There was no sign of real love or trust between its members and as Tanjiro Kamado put it, it was a fake bond that reeked of mistrust, fear, and disgust. One previous member told Daughter that each one of them came to Rui because they were afraid of the Demon Slayers and only played in his macabre game to survive.

Rui, however, took to great lengths to try to protect his family, even if most of them lived completely terrified of him. Nonetheless, due to being given Muzan's blood through him, each of the clan's members was all quite powerful demons; they were the strongest demons Tanjiro, Zenitsu and Inosuke had faced thus far in their lives and the clan was responsible for a great loss of life in the Demon Slayer Corps.

Initiation
As seen in Daughter's reminiscing of her past, upon being accepted and adopted into his family, Rui would first strengthen and augment the power of each new member by giving a single drop of his own blood and having them drink it. The blood would then painfully strengthen them and give them access to the various, spider-like Blood Demon Arts similar to Rui's own abilities. If they survived the process, Rui would then proceed to forcibly alter the newly adopted member's physical appearance to resemble his own before officially welcoming them into the family.

Role
In accordance with what Rui believes to be what constitutes a "family," he forces each and every member of his "family" to fulfill a specific role. "Father" and "Mother" are supposed to protect and give their lives for their "children" while the "Son" and "Daughter" are supposed to protect their "Younger Brother" with their lives without fail or compromise. Rui believed that these sacrificial-like roles are what truly make up a family, not at all understanding the concept of familial love that a true family has with each other and this being the true reason why they are willing to sacrifice themselves for their siblings/children. He forces his family members to go along with these roles by using terror, pain, and force to make them submit to him which deluded Rui into believing that these incredibly toxic relationships created through these violent methods are a true and powerful "bond" between them. Due to Rui's desire for a family, he would force the family members to perform typical Family-related activities such as making them gather around in a room and sit together, with empty plates and bowls of food presented to them to simulate a family dinner, as seen in Daughter's flashbacks.

Mount Natagumo Arc
After healing their injuries, Tanjiro Kamado, Zenitsu Agatsuma, and Inosuke Hashibira, are sent on a mission in Mount Natagumo. They stumble into a forest and encounter an injured fellow Mizunoto. Before he tries to warn them of the danger ahead, he is pulled away back into the forest, screaming he is attached to something.

Tanjiro and Inosuke venture into the forest, only to find it filled with spider webs and bodies of several Mizunoto Demon Slayers lying dead. They find Murata, a survivor who explains that their members suddenly started to erratically attack one another, resulting in a massacre. The three are soon surrounded by the moving bodies of their comrades which attack them. The trio defend themselves as Tanjiro realizes they are being manipulated and there are both corpses and living people controlled. Tanjiro discovers the threads are controlling their comrades which are placed by small spider demons around them.

Inosuke tries to kill them, but they are too many and his actions are in vain. Tanjiro pleads with him to detect the source of the spiders so they can avoid killing their comrades. Annoyed, Inosuke complies and detects Mother using his Beast Breathing, Seventh Form: Spatial Awareness. Murata stays behind to fight the controlled Mizunoto while the two went after the Mother.

On the way, they fight Ozaki who is controlled by the demon, her attacks made stronger by the manipulation of thicker threads. Her attacks take a toll on her as she is forced to fight to the point of her bones breaking. Tanjiro has her follow him and then turns around to launch her onto a tree's branch, entangling the threads. Inosuke follows suit with several Mizunoto becoming restrained. Angered by this, Mother pulls on the threads on their heads to break the necks of her victims and kills them on the spot, deeply enraging Tanjiro.

As Tanjiro and Inosuke continue on, the Mother unleashes her strongest puppet monster on the two of them. The monster proves to be incredibly strong and Inosuke sustains injuries in the process. With a combination attack, Tanjiro catapults Inosuke into the air, giving him the necessary power to cut the monster down. Inspired by this, Inosuke in turn launches Tanjiro to the Mother, who realizing her current life is full of torment and embraces her death with relief. Tanjiro notices her intentions and gently slices her head with Water Breathing, Fifth Form: Blessed Rain After the Drought so she feels no pain. Before dying, the Mother bids Tanjiro good luck and reveals a member of the Twelve Kizuki is on the mountain.

Meanwhile Zenitsu, who was left behind, ventures into the forest as he realizes that Nezuko Kamado, his crush, was inadvertently thrown into the mountain as Tanjiro was carrying her. Zenitsu follows after them, worried for her safety and wanting to protect her. On his way, he stumbles upon a terrifying sight of a house held by spider webs with hanging corpses on the threads. He is confronted by Son of the Spider Family, who tries to prey on him. Zenitsu, too scared to fight, tries to keep his distance from him and his spider minions, but is told by the Son that he was bitten by his minion which poisoned him. With the poison already in his body, the Son tells him he will suffer several symptoms before ultimately become a minion in 30 minutes, further horrifying Zenitsu until he falls unconscious.

Son finds Zenitsu weird but tries to consume him while he was unconscious. Zenitsu, now unconscious and unafraid, begins to retaliate and fight back, putting Son on alert. The Demon Slayer has flashbacks of his training days as he fights the spiders, with Son realizing Zenitsu is only able to employ a single technique and gloated his alleged victory. Zenitsu remembers his master saying that it is fine that he could only use one technique, which just means that Zenitsu has to work hard to master that one technique. He pushes himself to his limits and in an instant, defeats the Son Spider Demon. Weakened by the poison, he tries to control his breathing to the best of abilities and slow down its effects.

While in the forest, Tanjiro and Inosuke find Daughter and try to pursue after, who then summons Father to fight them while she flees. Father yells at the Demon Slayers to stay away from his family as he demonstrates his strength, almost landing a hit on Inosuke but is stopped by Tanjiro who tries to unsuccessfully slice his arm. Inosuke attempts the same but to no avail, and the two are pushed away by Father's fortitude. The two run away and Tanjiro strategically cuts a tree so it falls over the giant demon. He tries to seize the opportunity to use his strongest technique to kill the Spider Demon, but the latter manages to stand up and use the same tree to catapult Tanjiro away from them. He yells Inosuke to defend his life until he can return and help him.

Inosuke is forced to run but Father discovers his location due to his blood drops. He tries to escape but is then disgusted by how he started to think more strategically like Tanjiro, deciding to fight back against Father. His first slice sticks into the demon's arm until he uses his second blade to hammer the first, slicing through his thick skin. Father runs away with Inosuke in pursuit and he finds him in a tree. Inosuke gloats that Father is afraid of him only to witness the demon sheds his skin causing an increase in muscle mass and he regenerates his arm. Inosuke realizes he is outmatched and resigns himself to his fate until he remembers the words of the people he came across, including Tanjiro.

Regaining his senses, Inosuke tries to fight against Father but is overwhelmed by his speed and power. He finds an opening but his blades break when trying to cut Father's neck. The demon grabs him and tries to crush his head, with Inosuke receiving flashbacks of someone close to him as he is on the verge of death. Father's arm is then instantly sliced by Giyu Tomioka who timely saved Inosuke from death. The Spider Demon regrows his arm and tries to attack Giyu, but the Water Hashira instantly kills the demon chopping him to pieces, leaving Inosuke dumbfounded of Giyu's strength.

After being separated from Inosuke, Tanjiro finds Daughter being tortured by Rui. Rui tells Tanjiro to not witness their familial matters, however, Tanjiro protests that he can smell their true feelings and that there is no trust or love between them, just stagnant fear, disgust and mistrust, calling their familial bond fake. A Mizunoto appears and boldly claims he can kill a "kid demon" easily and climb the ranks that way, however, he is instantly mutilated by Rui's web. The pale demon then turns his attention to Tanjiro, angered by his claim that their family is a fake. Tanjiro realizes Rui is a powerful adversary and the two fight, with Tanjiro barely able to avoid Rui's threads. As Tanjiro finds an opening to attack Rui, his blade clashes with Rui's web, breaking it.

Tanjiro is able to narrowly evade, though he ends up sustaining a cut along his face. He tries to dodge Rui's attacks in order to get close and behead him, but cowers in the awe of Rui's threads' power and mobility. Rui asks Tanjiro if he will take back his previous statement and he once again refuses to retort his words. He retaliates by enveloping Tanjiro in a web of threads, but Nezuko suddenly appears to shield him from damage to his dismay. Witnessing this, Rui grows obsessed with having Nezuko for himself, believing her to be the perfect "sister" and yearning to develop a bond with her. Daughter protests she is Rui's "sister," but he furiously slashes her in response.

Rui scorns her and the rest of his family for not being able to fulfill the roles he ever gave any of them as he expected them to. Daughter begs for a chance at redemption and Rui orders her to kill the rest of the invaders to forgive her transgression, which she fearfully complies to. He then tries to negotiate with Tanjiro, wanting him to give up Nezuko to become his sister so that he can escape with his life. Tanjiro instantly refuses, believing Rui's idea of a bond forged through fear is nothing but a farce. Rui gloats that no matter what Tanjiro does, he can't defeat one of the Twelve Kizuki. As Rui explains his philosophy of roles to Tanjiro, he pulls Nezuko away from him with his threads, capturing her.

Tanjiro rushes to rescue Nezuko who furiously scratches at Rui's face, but is forced to stop to avoid Rui's threads. Tanjiro soon discovers Nezuko hung up in several threads as punishment for disobedience. As Tanjiro tries another charge, he stumbles trying to evade more threads and eventually finds himself at the mercy of Rui's beatings. Rui asks if he is trying to get close to behead him and gives him a chance to do so, revealing that his skin is harder than his threads before catapulting Tanjiro away with a strike. This infuriates Nezuko and Rui responds to her movement by tightening the threads to cut deeper into her body, causing her to fall asleep to regain strength.

Tanjiro tries to calm himself and attempts another charge at Rui using Water Breathing, Tenth Form: Constant Flux. Using the momentum of the rotation of several slashes, his attacks gain the necessary strength to cut through Rui's threads. Rui, however, activates his Blood Demon Art, strengthening his threads with his blood and trying to envelop Tanjiro on a thread. Before the attack succeeds, Tanjiro has a flashback of his father's Hinokami Kagura and the breathing technique used by him. Imitating his father's respiratory method, Tanjiro unleashes Hinokami Kagura: Dance, slicing through Rui's threads. Tanjiro relentlessly pursues Rui who tries to keep him at bay before the two are in the nick of hitting one another. Nezuko is called out by her mother's spirit who pleads her to protect Tanjiro, and she awakens her Blood Demon Art, igniting Rui's threads. Defenseless, Tanjiro seizes the chance to behead Lower Rank Five. Despite believing he had landed the finishing blow, the exhausted Tanjiro is shocked to discover that Rui had, in actuality, severed his own head with his threads. Now enraged at the attempt on his life and seeing both siblings are unable to retaliate, the demon draws a web in his hand and uses Blood Demon Art: Murderous Eye Basket, encircling Tanjiro in a closing snare of threads. As they are about to mince the Demon Slayer, they are suddenly destroyed by the Water Hashira Giyu Tomioka.

Rui, annoyed at the Demon Slayers' continual intervention, uses Blood Demon Art: Cutting Thread Rotation to slice his new opponent to pieces. The undaunted Hashira instead uses Water Breathing, Eleventh Form: Dead Calm and instantly slices the spinning eye of threads apart. Shocked to see his stRongest attack overwhelmed with ease, Rui tries to attack once moRe but is decapitated in the time he has blinked by Giyu.

Trivia

 * The Spider Family is one of the only mentioned united groups of demon actively working together and teaming up to fight, rather than operating on their own. The only other group is the Twelve Kizuki working for Muzan Kibutsuji.
 * The Spider Family was possibly the only large group of demons not under Muzan's command that was formed in the entire existence of the demon race, and it was implied by the Daughter Sister Demon that Rui had to obtain special permission from "that man" in order to form the "family," hinting that all other attempts of Demons creating large groups were all crushed by Muzan himself and later forbidden by him to ever form.
 * The Spider Family is most likely inspired by the Tsuchigumo, a race of spider-like yōkai in Japanese folklore. Like the Spider Family, the Tsuchigumo usually live in caves in mountains. It mainly feeds on humans, but also survives on animals around. The term Tsuchigumo is also used as a derogatory term for renegade local clans.