The Monster That Lurks Underground

 is an event taking place in the Tōjū Campaign. It focuses upon the conflict between the 11th Division 3rd Seat Ikkaku Madarame and his Zanpakutō Spirit, as well as the 4th Division member Hanatarō Yamada and the Tentacle Tōjū, in the catacombs beneath Seireitei.

Prelude
In the 4th Division barracks, Lieutenant Isane Kotetsu approaches Lieutenant Nanao Ise, asking her whether she would like to join her for tea. Nanao apologizes for using her desk, but Isane states that she doesn't mind. Isane announces that Lieutenant Momo Hinamori has made cookies for them, prompting Nanao to remember the last time she made cookies. Isane reassures her, showing her that they are ordinary cookies, moving to prepare the tea. However, she is unable to find any tea leaves, as Nanao comments that she recently bought them.

Isane reveals that many commonplace items have started to disappear, including rice and miso. Suddenly, Hanatarō bursts in through the door, proclaiming that his Hanamaru is missing. Isane shows a blurry picture of the culprit to Captains Shunsui Kyōraku and Jūshirō Ukitake, relaying that he broke into the barracks, stealing four of Hanatarō's chickens. Hanatarō states that they were fine yesterday, bursting into tears, as Nanao states that the culprit made away with the chickens without tearing through the fencing.

Isane suspects that the multitude of thefts are related, as Kyōraku wonders whether the culprit is a Tōjū. Ukitake asks Captain Retsu Unohana for her opinion, to which she states that they cannot rule it out. Unohana explains that they followed the culprit's tracks, which led to the underground waterways. Later, Kyōraku and Ukitake assemble several Shinigami, including Nanao Ise, Ikkaku, Hanatarō, and Hōzukimaru. He tells them that their job is to uncover the culprit, as Ikkaku wonders why he was summoned.

Entering the Catacombs
Kyōraku informs him that there is a definite possibility that the culprit is in fact a Tōjū. He wishes them luck, as Hōzukimaru states that this is the perfect way to pass the time. Ikkaku agrees, as Kyōraku wishes Nanao luck as well, prompting her to formally address him. As the task force makes their way into the dungeons, they are met with a parade of support from other Shinigami, as Isane opens the entrance for the four people. As they make their way through the passages, their locations are displayed on a map.

Kyōraku and the other captains approach Akon, who informs them that such is the case. He also comments under his breath that they are using this as an excuse to further document the underground waterway's passages. Unohana informs him that it was the perfect opportunity, as Isane wonders whether they are alright, to which Kyōraku reassures her. While Ikkaku leads the group down the dark passageway, Hōzukimaru comments that the area is creepy, which Hanatarō confirms.

The 4th Division member states that there are a lot of scary rumors about the waterways, as Hōzukimaru asks him to elaborate. As he states that the voices of ghosts can be heard, Nanao dismisses his story, claiming that there is no such thing as ghosts in Soul Society. However, as she says this, a droplet of water splashes onto her back, causing her to panic uncontrollably. The others ask what is wrong, to which she claims that there is something on her back.

Hanatarō reassures her that it was just a water droplet, claiming that water drips down from the ceiling. He calls Nanao a scaredy-cat, as he moves forward, tripping upon a thin strand of wire. An explosion hurtles throughout the area, as Hōzukimaru rescues the two. Hanatarō asks what occurred, to which Ikkaku claims that they are unwanted guests. Nanao observes that the enemy must be lying in wait nearby, telling them to proceed with caution from here on out.

As they proceed further into the cave, Ikkaku notices a trap lying on the floor. Hanatarō tries to process this information and attempts to walk around the trap spot. However, he steps upon a section of stone, which recedes into the ground, alerting Ikkaku and the others. Hanatarō flinches, but is surprised when nothing happens. Ikkaku tells him not to take a step out of line, as Nanao tells them to stop fighting. She asks them whether they can hear the sound of water, but they cannot.

The Tōjū Appears
As they stare at the clearing, a torrent of water gushes forth, sending the task force into a frenzy. As they run away from the voluminous column of water, Nanao tries to explain what occurred, but Hōzukimaru tells her it is not the right time for an explanation. They are then swept up by the tidal wave, and dragged out into an open pool section. Hōzukimaru asks whether Hanatarō is alright, to which he says he is fine. As Nanao examines the area, Ikkaku berates him for not knowing where they are.

Underneath the water, a form moves closer towards them, as Nanao tries to keep them from panicking. Tentacles enclose around the four, as Ikkaku proceeds to further insult Hanatarō, before being dragged under the water. They all call out for him, as the air bubbles disappear from the water's surface. Hanatarō tells Ikkaku to cut it out, believing it to be a joke on his part. As Hanatarō laughs uneasily, an enormous form can be seen from underneath them.

As the tentacles emerge from the water, the resulting force sends them hurtling backwards. Ikkaku's body becomes visible, being constricted by a multitude of tentacles. Hōzukimaru proceeds to assist his Shinigami master, despite Ikkaku's protests. However, the Tōjū disappears alongside Ikkaku, as a tidal wave pushes them away. As they wash up by the waterway, panting heavily, Nanao wonders what that creature was, as Hanatarō sobs over Ikkaku sacrificing his life to save them.

The Hooded Figure
Hōzukimaru tells him not to worry, stating that, as Ikkaku's Zanpakutō, he can sense Ikkaku's presence. This statement reassures Hanatarō, as he smells an odor wafting from across the tunnels. They all wonder what it is, as Hanatarō follows the trail of smoke, leading him to the culprit's hiding place. He wonders what the place is, and sees several skewers roasting over a fire. He exclaims, rushing over to the fire, claiming that it cannot be true. He wails in agony, as Nanao recognizes the sake bottles on the table to be that of her captain's.

Hōzukimaru realizes that all of the 4th Division's stolen goods are accounted for, as they hear a rattling noise from nearby. A cloaked figure appears before them and is startled by their presence, proceeding to flee from their company. Hōzukimaru chases after it, appearing directly in its path. He tells the figure that he is not escaping, as it runs in the other direction. They both call out to Hanatarō, as they proclaim that they are in pursuit of the culprit. As they race in Hanatarō's direction, the figure stumbles out of his cloth.

The figure then crashes uncontrollably into Hanatarō, stumbling onto the floor. Later, the doll-like figure begs for his forgiveness, as Nanao wonders what it is. Hōzukimaru tells him to raise his face, as Hanatarō seemingly recognizes it. Nanao berates him for acting like he knows something, prompting him to state that something is familiar about him. Hōzukimaru cuts to the chase, demanding what he did to Ikkaku. Nanao interjects, asking what he is doing here. The figure frantically apologizes repeatedly to them.

Identity, The Culprit is Revealed!
Their interrogation is interrupted by a huge explosion, in which Ikkaku emerges from a hole in the wall. He stumbles across the ground, before crashing into the area. Hanatarō and the others are relieved to see him as the Tōjū roars, appearing before Hōzukimaru and Nanao. He comments that the monster has showed up, telling Nanao to protect the others. Hōzukimaru states that he will avenge Ikkaku's defeat, materializing his Shikai in his hands.

He then releases his Bankai, Ryūmon Hōzukimaru, much to the Tōjū's surprise. As he throws his blade at the Tōjū, he is surprised that it is unable to affect it. As he retracts his blade for a second attack, the Tōjū's tentacles entwine around him, lifting him effortlessly into the air. Nanao intervenes, using Hadō #31 Shakkahō, which she fires directly at its head. The resultant blast causes the Tōjū to lose its grip on Hōzukimaru, flinging him into the dark recesses of the waterways.

They call out to him, as Nanao asks Hanatarō why he cannot fight. He claims that his Zanpakutō is solely for healing, and what's more, he left it in the barracks as he saw no need for it. The Tentacle Tōjū then grabs Nanao, who screams out loud as Hanatarō looks on, being unable to do anything. He wonders what he should do, as the figure notices Ikkaku's katana lying upon the ground. It clutches his katana, plunging it into Ikkaku's body, much to his surprise.

Ikkaku starts to convulse from the wound, as Hanatarō fears for Ikkaku's life. However, the bar on the katana glows red, with the red glow also appearing upon the figure's head. Hanatarō realizes whose Zanpakutō it is, as an enormous cannon emerges from the figure's chest, as Hisagomaru releases Akeiro Hisagomaru, blasting the Tōjū with a stream of red energy, which proceeds to disintegrate it, as Nanao emerges unharmed, splashing down into the water below.

Aftermath
Back at the 4th Division barracks, Kyōraku is confused that the culprit was Hanatarō's Zanpakutō, which he confirms. Nanao asks him why he didn't tell them that earlier, to which he claims that he didn't realize it himself. Kyōraku comments that that is symptomatic of the 4th Division's 7th Seat's behavior, as Nanao explains Hisagomaru's backstory. She states that he materialized alongside the other Zanpakutō, but he never held any disdain for his master, and tried to go back to his sword. However, when he was unable to, Hisagomaru believed that his master had discarded him. As a result, he relocated to the underground waterways in order to live a solitary life. Nanao recalls that, after the Tōjū was killed, Hisagomaru was exhilarated that his master had not rejected him. Hōzukimaru states that he hasn't returned to normal either, as Hisagomaru fell backwards in relief. Hanatarō starts laughing, as Kyōraku inquires about the chicken skewers. Hisagomaru states that he merely kept the chickens for company.

Hanatarō reassures him, stating that he is not alone. They then embrace each other, as Shinigami and Zanpakutō. However, Kyōraku breaks the mood by asking where Hisagomaru is, to which Hanatarō informs him that, on their way out, they got separated. As Hisagomaru crouches beside a rudimentary fireplace, he wonders where everyone else is, tears streaming down his face.