Kodoku (蠱毒, worm toxin), also called kodō (蠱道, worm path), kojutsu (蠱術, worm technique), and fuko (巫蠱, divination worm) is a type of poisonous magic found in Japanese folklore. It is the Japanese derivative of the Chinese Gu magic.
To create kodoku, sorcerers would mix several insects in a jar, and let them kill one another until only one survived. The fluids of the insect that survived would be used to poison an individual with a curse that would control them, cause them misfortune, or kill them. The remaining insect could also be used as a sort of "luck charm" granting the one who performed the ritual great wealth. In return the owner is supposed to feed the bug. Neglecting to do so would enrage the insect, if the owner does not equivalently repay the insect by placing all his or her riches beside a road, plus interest in gold and silver, the insect would devour the home owner. Therefore, this ritual could also be used as a death curse by giving the riches to an ignorant individual. The term "kodoku" can also be applied to the spirit which is the incarnation of this particular magic (which usually appears in the shape of a worm or other animal). The technique was used in the Nara Period.