Shūtai

A Shūtai (醜態; Japanese for "abomination") is a rare occurence that results in a Zanpakutō spirit consuming its owner and fusing them inside said spirit.

Overview
Normally sentient beings, Zanpakutō spirits work in harmony with their Shinigami owners. This is because Zanpakutō spirits are manifestations of the Shinigami's soul and power. However, while beyond rare, a Zanpakutō can - instead of living in the Shinigami's inner world - manifest themselves and literally devour their Shinigami. Since Zanpakutō spirits need the Reiryoku to effectively use their powers and exist; by consuming the Shinigami, they can fuse them to the Zanpakutō spirit, thus gaining their Reiryoku.

Due to the joining of two spiritual bodies, the fusion cannot be undone by any known methods. The Zanpakutō can now remain in its manifested form and the Shinigami inside is in a comatose-like state.

Abilities & Powers
A Shūtai can possess numerous types of powers; multiple abilities that it didn't have when it was living in the inner world of its Shinigami owner.

The Shūtai can like numerous spirits increase the strength of it's Reiryoku.

Furthermore, they may develop their Kidō process and produce new Kidō.