Bleach has a variety of characters with different races and powers, Soul Reapers being the most common. They are known to wield a weapon called Zanpakuto, which is used to carry out their missions of not only defeating their enemies but also helping lost souls find their way to the Soul Society. In its sealed form, a Zanpakuto looks like a regular sword. However, once released, every sword has a unique appearance and ability.
The released form of a Zanpakuto is called Shikai, which every Soul Reaper has to master before joining the Gotei 13. Beyond Shikai is a level called Bankai, known to be the ultimate form of a Zanpakuto that only a select few can possess. Those who are known to master Bankai are either captains or someone at the same level as a captain. However, a lesser-known fact among Bleach fans is that there is a final stage of Zapakuto that surpasses even Bankai. In fact, it’s so rare that even in the final arc, the series hasn’t yet introduced it.
Shukai is the third and final form of a Zanpakuto that displays the powers of the Soul Reapers as well as the Zanpakuto’s true form. This form has been mentioned in side stories but not in the anime. Unlike with Bankai, activating a Shukai not only gives the Zapakuto a new name but allows the Soul Reaper to receive one as well. This is because Shukai not only completes a Zanpakuto but also the Soul Reaper who wields it. According to legends, thousands of years before Gotei 13 was founded, a class of Shinigami possessed an innate ability to use Shukai.
However, for some reason, after the formation of the Gotei 13 and the present Spiritual World, Shukai users stopped appearing, turning those legends into myths. Apart from different appearances, a key difference between Shukai and Bankai is the power increase; Shukai is approximately ten times more powerful than Bankai. Considering that Bankai increases the power of a Soul Reaper about five to ten times, Shukai is fifty to one hundred times stronger than a Shikai.
Furthermore, unlike Bankai, Shukai cannot be broken, suggesting that it ensures the victory of the person wielding it. A major drawback of this power is that Shukai’s release is known to be finite. Unlike Bankai, which can be used indefinitely, Shukai can only be used a specific number of times the wielder will eventually lose the ability to use it ever again. One Shukai user lost this ability completely after only four months of his first usage.
The fundamental characteristics and requirements of Shukai aren’t that different from Bankai. There are three major requirements for one to reach the final stage of Zanpakuto. First and foremost, the wielder must have not only achieved but completely mastered their Bankai. This is similar to the requirement of Bankai, which can only be achieved by fully mastering Shikai. Another requirement is the complete manifestation of the Bankai spirit, which is different from a Shikai spirit.
Just as any Soul Reaper first needs to manifest their Shikai spirit before learning how to use Bankai, Shukai’s requirement is to manifest the physical spirit of the user’s Zanpakuto’s Bankai within the real world. The final and most important requirement for one to master Shukai is to have genuine affection for Zapakuto. The creator of Zanpakuto, Oetsu Nimaiya, stated that Soul Reapers who hate or don’t have genuine affection for their Zanpakuto can never achieve the final stage. Furthermore, he claimed that the mere act of achieving this stage is only a coincidence an occurrence so rare that it only happens a handful of times over thousands of years. However, simply meeting all three of the above requirements isn’t enough to attain Shukai.
There was once a ritual known as Ketsurui that only Oetsu could administer in a place called Koruihora — a cave located in the Royal Palace city of Tenkyuden. The unique powers of the ritual allowed the Bankai spirit to completely split from the Soul Reaper. The spirit then materialized into an entity that was the mirage of the fears and nightmares of a Soul Reaper and their Zanpakuto. The two must work together to defeat that entity by fighting together as one being. To do that, the Soul Reaper and their Zanpakuto must understand each other completely and learn about their strengths and weaknesses. The ritual was by no means ordinary since it was potentially deadly. Once the ritual was started, the Soul Reaper would either obtain Shukai or die. Even Oetsu himself was incapable of stopping this ritual once it began.
Needless to say, it’s only fair for the creator of Zanpakuto to have mastered Shukai. Oetsu was able to join Squad 0 for creating the Zanpakuto, and he became the head of the Gingakei Hooden, one of the cities located in the Soul King Palace. Despite being the man who invented Zanpakuto, his Shikai, Bankai and Shukai remain unknown to this day. Although it is mentioned that he had this ability, he was never shown to have used it. Another known user of Shukai is Hyakuoku Getsueikirite, an ancient Sozoshin (Creation Spirit) and the founder of the Getsueikirite Clan. He is known to be the first user of Shukai, who used that power to defeat an ancient monster.
The only other user of Shukai was Raian Getsueikirite, a former Soul Reaper who left the Soul Society of his own accord and started living in the Human World. Before leaving the Soul Society, he was the captain of Squad 10. He made Isshin Shiba his third seat soon after he became the captain. He was the direct descendant of Hyakuoku Getsueikirite. There’s nothing mentioned about his Shukai, but he had dual Zanpakuto in the form of twin Nordic axes. One ax represented his Soul Reaper powers, while the other represented the Hollow within him.