Kodoku

Kodoku (蠱毒) is a work of sorcery; it is a magical art which is to close in a vase, venomous insects or lizards to make them kill each other. The only survivor turns into a creature called kodoku. Naraku uses this sorcery with demons imprisoned in a cave. The survivor absorbs all the other demons, and then in turn be absorbed by Naraku, who needed a new body, as the former had been severely damaged by Kagome to the castle.

Inuyasha present in the cave, fights with the demon survived in danger of becoming the kodoku him, but luckily, Kikyo too present there, destroys the seal, saving Inuyasha, but allowing Naraku to get his new body. Miroku is the first to discover that the battle of the demons in the cave was a sorcery to create the kodoku.