Kodoku
, also called , , and  is a type of poisonous magic found in Japanese folklore. It is the Japanese derivative of the Chinese  magic.

It is said to have been widely used in ancient China. It is not clear for how long it has been used, but scholars of Chinese characters such as Shizuka Shirakawa, who advocates the importance of magic in ancient times, have found traces of poison in the oracle bone script of the Yin and Zhou dynasties. The earliest record of worms is in the Geography of the Suishu reads:

To create , 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  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.

In fiction

The technique plays an important role in the first part of the 1985 historical fantasy novel Teito Monogatari. The protagonist Yasunori Katō is a master of kodoku magic and uses it to manipulate his victims.  The spirit is represented as an insectoid worm (腹中虫, fukuchu-mushi) which lives in the stomach of the victim.
The term "spiritual kodoku" is used in the anime Ghost Hunt for a curse technique in which spirits are trapped like insects are in traditional kodoku practice. A dominant spirit devours the weaker spirits until it has enough power to kill the target of the curse. The only way to stop the curse is to "feed" the spirit as compensation.
 In InuYasha the main antagonist Naraku creates a kodoku inside a mountain to gather and merge hundreds of yōkai to form a new body for himself.
 Kodoku Experiment is a science fiction manga from Yukinobu Hoshino where a kodoku is being created on a planet filled with ferocious monsters.
 Kodoku is used in the anime Fushigi Yûgi (Episode 18) as a drug to change the personality of one of the major protagonists, Tamahome.
 The Kagewani from the anime of the same name is a shadow creature that was created through kodoku using several different animals instead of insects.
 In Ōsama Game: Kigen, a manga prequel of Ōsama Game, the culprit behind a murderous game is subsequently revealed to be a virus, created  through kodoku by Natsuko Honda's ancestors.
 The idea of Kodoku is a central motif to the plot of the manga BIOHAZARD Heavenly Island, where it is used by the main antagonists to develop a new B.O.W.
 In the web novel Re:Monster, Kodoku is used by the main protagonist as a method to create a more powerful army, from pitching many different summoned creatures against each other inside a hole, and letting them "evolve" by themselves. 
 In Mob Psycho 100, one of the minor antagonists, Matsuo, uses Kodoku to merge evil spirits to create powerful "pets".
Hunter x Hunter succession arc is based on this ceremony.
 The Ao No Haha manga is entirely based upon the Kodoku Magic, there it is used to make stronger medicine by using a different "pot".
 Kotoribako and Ryoumen Sukuna (elements of Japanese folklore and Kowabana, Japanese scary stories usually spread by rumors and forums) may have been created by using some variant of the Kodoku Magic.
 In the Manga series Yu-Gi-Oh, the Millennium Items are created by sacrificing a certain number of "evil" (poisonous) people, and are clearly defined as a curse, each of them putting a weight on strong-minded users. Also, the Millennium Puzzle can be deemed a sort of Kotoribako (a difficult puzzle box created as a curse containing the remains, here the soul, of an individual, a young one moreover).
 In the Manga series Dokumushi the main characters are forced to participate in a kodoku involving humans, the only person who could be freed would be the one who survived by murdering and committing cannibalism with the other participants.
 In the Manga and Anime series Blue Lock the titular Blue Lock is a prison-like facility where three hundred talented high school soccer players from all over Japan are forced to participate in a soccer kodoku, the overall winner who is explicitly called the "sole survivor" will earn the right to become the national team's striker and those who lose will be banned from joining the team forever.
 In the Manga and Anime series Dark Gathering one of the main characters forces captured spirits in a kodoku in order to create powerful spirits.
 In the Manga and Anime series Jujutsu Kaisen Sukuna partacts in a bath that was traditionally a ritual used to protect family heirlooms by turning them into cursed tools. It required soaking in a solution of cursed energy formed from crushing and straining venomous creatures through the process known as kodoku. Uraume was able to reconstruct this ritual using cursed spirits in the Zenin clan's disciplinary pit. Uraume froze the cores of curse spirits before slicing them apart and filtering them. Sukuna is using the bath to become closer to evil, which submerges Megumi's soul into submission. 
 The Japanese visual kei band Kiryū has a music video named Kodoku which describes the practice of inflicting curses through poisoning.
 In volume 10 of the light novel series Reign of the Seven Spellblades, Tim Linton describes his childhood in terms of the "bug urn" used to produce poisons in the faraway land of "Chena", which he calls "halfway to a full-on curse". The Linton clan's magical tradition is to collect magically talented orphans, poison them, and force-feed the ones who died to the survivors, repeating the process until only one strong, poison-themed mage child is left, whom they raise as their heir.

See also
 Kōshin

References 

Japanese folk religion
Poisons
Magic (supernatural)
Insects in religion

ja:蠱毒