Kaien Shiba, Jūshirō Ukitake & Rukia Kuchiki vs. Metastacia

 is a fight which took place several decade ago between Kaien Shiba and Metastacia after the latter killed Miyako Shiba, with Jūshirō Ukitake and Rukia Kuchiki being forced to fight as well once the battle takes an unexpected turn.

Prelude
5th Division Captain Sōsuke Aizen creates Metastacia in an attempt to bring into existence a new kind of being with the powers of both Shinigami and Hollows that can achieve new heights of power. With the aid of his unique abilities, Metastacia proceeds to kill many Shinigami, including Miyako Shiba, the wife of 13th Division Lieutenant Kaien Shiba, when she is sent to purify him. After Miyako's remains are brought back to the barracks, a mortified Kaien stands in front of them while his captain, Jūshirō Ukitake, tells him her unit was annihilated.

With a shocked Rukia Kuchiki standing behind Kaien, Ukitake explains that that they barely know anything about the Hollow who did this due to none of the Shinigami it attacked escaping and promises to go after it in two days once they have assembled a combat unit, only to be interrupted when Kaien attempts to depart. As Ukitake tells him to wait and reminds him they do not know anything about the enemy, Kaien claims to know all he needs to know and not care about the Hollow's abilities before demanding to know if Ukitake expects him to sit here and do nothing while the combat unit handles this.

Ukitake clarifies they know the Hollow nests in one place to hunt for prey instead of migrating, as well as where the nest is. That night, Rukia, Ukitake, and Kaien stand on a tree branch outside a cave, where Metastacia wanders out and asserts he can smell prey before laughing wildly. Rukia grips her Zanpakutō's handle and tells Kaien she will go first, but Kaien requests Ukitake let him go alone, to her shock. With Ukitake permitting this, Kaien leaps onto the ground in front of Metastacia, who expresses delight at him going first. Noting he has to ask Metastacia something before they fight, Kaien questions how many Shinigami he has eaten.

As Metastacia admits that he does not recall, Kaien questions if he has ever regretted it, prompting Metastacia to call this a stupid question and assert that he has feelings just like Kaien before claiming that there is not a single night where he does not regret eating Shinigami, only to clarify that right now he is regretting not eating the rest of Miyako last night while laughing maniacally.

Battle
Kaien solemnly acknowledges this and draws his Zanpakutō as he exerts his Reiatsu, leading Metastacia to mentally urge him to come over so he can eat Kaien as well. Suddenly, Metastacia finds two of his left arms severed in a spray of blood while Kaien grabs one of his tentacles and stands on his back. Though surprised by Kaien's speed, Metastacia gleefully observes that the former touched him. Attempting to release his Shikai, Nejibana, Kaien is shocked to find his Zanpakutō shattering while Metastacia reveals that the first Shinigami to touch his tentacles each night has their Zanpakutō destroyed. Stunned by this, Kaien is slashed by one of Metastacia's tentacles and wonders how he could have such a power before being sent hurtling away. Frantically calling out to Kaien, Rukia attempts to draw her Zanpakutō and leap down to help him, but is stopped by Ukitake.

With Rukia telling him to let her go so they can save Kaien, Ukitake inquires what she thinks will happen to Kaien's pride if she does this and elaborates that aiding Kaien now will kill his pride forever even if they saved his life. Despite Rukia asserting that pride is meaningless compared to life as Metastacia continues to attack Kaien, Ukitake explains that there are two types of battles and one must know whether each battle they fight is one to protect life or to defend honor. As a stunned Rukia sees him sweating in concern while Kaien rips off one of Metastacia's tentacles, Ukitake details how Kaien is fighting for the honor of his men, his wife, and himself above all else before concluding that Rukia should let him fight this battle alone.

Ukitake asks Rukia to do so even if she thinks of it as foolish pride, prompting a distressed Rukia to remove her hand from her Zanpakutō. Down on the ground, Metastacia praises Kaien for surviving this long without a Zanpakutō, and when Kaien asserts that his Kidō is more than sufficient for this fight, Metastacia criticizes him as arrogant and bemoans having to do this again today, only to claim that he has no choice with Kaien insulting him like this before discharging the tentacles on his back toward a shocked Kaien. While Rukia calls out to Kaien in concern, the tentacles enter Kaien's outstretched arm at the wrist and funnel into his veins, leaving the remainder of Metastacia's body to fall apart, and Metastacia assumes control of Kaien's body, causing his eyes to turn black, his skin to turn green, and his face to gain the markings of Metastacia's mask as Metastacia looks over his shoulder and menacingly asks a frightened Rukia if she called his name.

Questioning why Rukia keeps calling Kaien's name like this, Metastacia inquires if she is worried about him because he is dear to her while looking at Rukia with his pitch-black eyes. As Rukia reels back in fright, Metastacia leaps up to her and declares that he will eat her first if this is the case, only for Ukitake to appear between them and intercept Metastacia's bite with his Zanpakutō. Upon being ordered by Ukitake to get out of here if she does not want to die, Rukia frantically leaps away from the tree branch and tearfully wonders why this is happening.

Behind her, Ukitake pursues Metastacia, who wonders why he is not attacking and theorizes that Ukitake is trying to think of a way to drag him out of Kaien's body, only to proclaim that this is pointless before revealing that, due to both him and Kaien being spiritual entities, this is a complete fusion and one they cannot be separated from, unlike Human possession. However, while Metastacia proclaims that he will spend all night devouring Kaien's Soul from the inside, he is interrupted when Ukitake slashes through the side of his neck, leaving him stunned and bleeding profusely, and calmly asserts that he will simply kill Kaien along with Metastacia if this is true.

Metastacia is shocked that he would kill his subordinate with his own hands as Ukitake declares that he will not allow Metastacia to have Kaien's body. Suddenly, Ukitake coughs up blood and curses this happening at such a time, prompting Metastacia to leap forward and attack him with tentacles emerging from his arm. Despite this, Metastacia merely wounds Ukitake and leaps past him, leaving a confused Ukitake to see Rukia standing in the woods behind him and demand to know why she came back. With Rukia mentally admitting that she has no plan and simply knows that she cannot run away without saving Kaien as Metastacia hurtles toward her, Ukitake orders her to kill him because he is not Kaien anymore, and as she sees tentacles emerging from Kaien's mouth and right eye, Rukia lifts up her Zanpakutō, causing Metastacia to impale himself on it.

Aftermath
As rain begins to fall around them, Kaien regains control of his body and thanks Ukitake for letting him fight while the latter approaches from behind. When a stunned Rukia addresses him, Kaien apologizes for getting her into this mess and hugs her as he observes that she must have been scared. Kaien thanks Rukia for doing this so he can leave his heart behind, and as Kaien's hand falls from her back and begins to disintegrate, Rukia wails in sorrow and mentally asserts that there is nothing to thank her for, with her running away because she was afraid to fight him, coming back because she could not stand her own cowardice, and using her sword because she could not bear to see him suffer, concluding that she did it all for herself and that she is pathetic.