Dourei

Dourei (どうれい, Dōrei) are artificial humans, initially created by the mercenary group known as the Black Crown, who posses a wood-like Hiden. These being were initially people, or as Blsck Grown called them, servants, that would automatically explode.

Background
During the Kage Summit, these people, were sent by Black Crown, to use their own life to effectively blow themselves up, in attacks to take civilians down with them. Shingi, the Takikage, managed to grab the DNA product of the aftermath, otherwise a pieces of the bark that the servants left behind. Returning to his own lab, Shingi managed to throughly scan the data, to the point he was finally able to create artificial beings of his own. He also managed to tweak them far beyond what the Black Crown could have expected, thanks to the help of Yggdrasil.