The 13 episodes of Bleach's "Thousand-Year Blood War" arc saw more deaths than the rest of the series combined, as the King of Quincy returned after a thousand years to wreak havoc on the world. What's more surprising is that, despite having annihilated the Quincy millennia ago, they now don't stand a chance against the villains. On top of that, the Soul Reaper Captains are powerless after their Bankai is stolen.
As Nodt steals Bankai from Byakuya Kuchiki, Bambietta Basterbine steals from Sajin Komamura, Cu Dang steals from Toshiro Hitsugaya, and BG9 steals from Sui-Feng. The situation worsens when the stolen powers are used against the good guys, causing significant damage. When Byakuya is hit by his own Bankai, he barely survives. Fortunately, the situation improves when the most brilliant Soul Reaper, Kisuke Urahara, discovers a way to recover the stolen Bankai.
Quincy of Bleach Steals and Uses a Soul Reaper's Bankai
The Quincy use a device called the Wandenreich Medallion to steal a Soul Reaper's Bankai. They have the innate ability to accurately identify the intensity of anyone's spiritual energy, allowing them to differentiate between high and low points; this result in the Medallion's ability to steal Bankai. So far, Bleach has featured one Medallion capable of stealing only one Bankai and then using it almost instantly. A Soul Reaper normally requires ten years of training or more, but the Quincy can use the stolen Bankai to its full potential without any issues.
In some cases, the Medallion will only activate once the incantation is complete. Once it's activated, a circle of energy with the insignia of a Quincy cross appears and quickly becomes a giant cross of light raining down from the seal. This instantly seals away the powers of a Bankai user. It leads the Soul Reapers to misunderstand that Bankai can only be stolen, therefore, they are caught off guard upon seeing their powers being used against them. Byakuya Kuchiki reveals his Bankai not knowing it can be used against him and faces an overwhelming defeat.
Is There a Flaw in the Wandenreich Medallion?
As Bleach's "Thousand-Year Blood War" arc showed, the Medallion doesn't work in all cases since Ichigo Kurosaki's Bankai remained with him. Soul Reapers with powers that exceed the Medallion user's cannot have their Bankai stolen. Furthermore, Yamamoto stated that his Bankai couldn't be taken a thousand years ago because he hid its real powers. Therefore, the Quincy cannot steal something they don't understand; they require their target to activate the Bankai first and must be within a certain range.
Because the Captains do not activate their Bankai unless absolutely necessary, the Quincy force the Soul Reapers into a corner so the Captains have no choice but to do so. However, the risks involved go both ways; the Quincy become unable to use their Vollständig until they relinquish said Bankai. Both these powers exist as a reflection of an individual's soul, thereby forging a singular connection between them. Furthermore, the Bleach Silver spinoff shows that smashing a Medallion holding someone's Bankai inside it will cause it to automatically return to its original owner.
Why is it impossible to steal Ichigo Kurosaki's Bankai?
Even in Bleach's expansive world of widespread realms and powers, protagonist Ichigo Kurosaki is a rare being. He has the powers of a Soul Reaper, Hollow and Fullbringer, and is even half-Quincy. As such, his powers vary greatly, allowing him an upper hand against any enemy. Asguiaro Ebern, an Arrancar soldier in service to the Wandenreich, tries to steal Ichigo's Bankai but fails miserably. One reason for this is Ichigo's inner Hollow, which contaminates an individual's spiritual pressure and serves as a fatal poison to the Quincy's Spiritual makeup.
Quincy are unable to take in Hollow Reiatsu without their souls being destroyed in the process. This leaves them unable to steal any Bankai that has the spiritual energy of a Hollow. Ichigo's Zanpakuto isn't real when he fights against Ebern; rather, it's a manifestation of his Quincy powers disguised as a Soul Reaper's. Furthermore, the Medallions aren't designed to steal Quincy powers. Since Ichigo's Shikai and Bankai are his Quincy abilities, it can be said that the Medallion fails to capture his Bankai because Ebern cannot understand its fundamental properties.
How the Captains of Bleach Recover Their Bankai from the Quincy
Kisuke Urahara has the greatest intellect in Bleach's Soul Society, and even the mighty Sosuke Aizen is afraid of him. Being able to create a horrifying object like the Hogyoku demonstrates his unrivalled intelligence. There are several layers of research and testing to ensure the Soul Reapers' victory by reclaiming the Captains' Bankai. Urahara exploits the Medallion's weakness after determining that it cannot unite with Hollow powers. He studies the Quincy Medallions and their abilities before creating the Shin'eiyaku pills.
These pills cause temporary Hollowfication -- effectively making the Bankai a poison for the Quincy -- and they can be consumed by the thieved Soul Reaper through any means of physical contact. The Bankai gradually returns to the owner, making them Hollowfied. For example, Toshiro Hitsugaya gained a mask over his left eye when his stolen Bankai returned to him. This is how all the Captains get their Bankai back and are able to put up a real fight against their powerful Quincy opponents.