Warring Souls Period

The Warring Souls Period (戦霊時代, Senrei Jidai; literally "era of the warring souls") was a period in the history of Soul Society, the longest and bloodiest era in which there existed only two factions: those that supported the King of Souls and those that sought their own path of conquest. Many significant beings, such as Genryūsai Shigekuni Yamamoto, Kenpachi Unohana, and the four Yonkō, had made their name and their power well known during this time; renowned as heroes, villains, and the like. Following the defeat of the Quincy during the Quincy Blood War, the Gotei 13 had achieved total dominion over the world of spirits and were recognized by the King, who had later delegated authority to the Central 46 and left from the Soul Society.

Behind the Scenes
The name for this era is derived from the Sengoku Period (戦国時代, Sengoku Jidai), or Warring States Period, which was a timespan in Japanese history lasting somewhere around three centuries; during which time, the Ashikaga Shogunate collapsed and Japan was torn into various factions in the midst of a bloody civil war. There is also an era in Chinese history as well that carries the same aforementioned name.