Naigai

Naigai (亡い骸, Dead Body) were invented by Muramasa Neikan for the use of the Impero Nascosto. Muramasa touts this as the "completion" of and  research.

Overview
Naigai are gigai that use to come alive. The soul candy, however, has been modified. Thanks to Akujin having stole Genryūsai Shigekuni Yamamoto's Bankai, Zanka no Tachi, early in the War of the Worlds, combined with the reishi of a certain person, Muramasa can bring that person back by tying their soul to the candy with their reiatsu and Zanka no Tachi's Kaka Jūman Okushi Daisōjin power.

The reanimated dead retain their personalities and abilities, but are under the complete control of their Naigai's creator. Furthermore, there are only two ways to defeat these Naigai; destroy their soul candies, or destroy the Bankai Stealing Device that holds Yamamoto's Bankai.

Influence
This invention was heavily influenced by the Summoning: Impure World Reincarnation ability that is introduced in the Naruto series and is made to be similar to it.