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. So far, only Ichigo Kurosaki, Renji Abarai, and Tōshirō Hitsugaya have successfully defeated their Zanpakutō, while Byakuya Kuchiki has chosen to join Muramasa.

Trivia

 * During the Zanpakutō arc, Renji Abarai has a flashback of his fight against Szayel Aporro Granz when he fights Zabimaru, indicating that it is set after the current manga storyline. This might cause a conflict with the manga, as the future of the main characters is unknown (however they may change this in the anime).
 * 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 belonging to lieutenants have yet to be seen (ex. Isane Kotetsu's Itegumo).