Zanpakutō Unknown Tales arc

The is the term used to refer to all the events that took place during the anime-exclusive story arc focusing on the rebellion by the spirits of the Shinigami Zanpakutō.

Outline
Muramasa, a mysterious Zanpakutō spirit with the ability to set other Zanpakutō spirits free from their bonds to their respective owners, instigates an uprising against the Shinigami, questioning their right to rule over the Zanpakutō. By forcibly unleashing and warping their inner desires, Muramasa convinces the Zanpakutō spirits to join him against their former masters. The Zanpakutō rampage across Soul Society, causing vast amounts of damage and casualties.

It has since been revealed that Zanpakutō can be restored to their owners by said owners defeating the individual spirits. While several Shinigami have successfully defeated and regained control of their Zanpakutō, including Ichigo Kurosaki, Renji Abarai, and Tōshirō Hitsugaya, Byakuya Kuchiki chose to join Muramasa.

Trivia

 * Informally referred to as the Zanpakutō Rebellion arc.
 * Tite Kubo involved himself in the creative thinking of this arc, particularly when it came to the appearance of the Zanpakutō spirits themselves.
 * Despite Muramasa claiming to have "freed" all the awakened Zanpakutō spirits in Soul Society (minus Yamamoto's Ryūjin Jakka), several Zanpakutō spirits have yet to be seen (such as Isane Kotetsu's Itegumo).