A kodoku (蠱毒) means "worm toxin" in Japanese. It is similar to Chinese Gu Magic. Gu Magic uses living creature. You place poisonous bugs, such as spiders, scorpions, or centipedes, into a jar. Several months later, you dig them up and perform a death curse with the one survivor, who has devoured the others concentrated the poisons of the others into its body. It can be used with any animal is a confined space, even dogs, cats, etc.
A kodoku works in the same way, even with spirits as seen in the Ghost Hunt anime. With insects, it's easier to control and hide. The bug is supposed to bring wealth to the household, but must be feed a human life in return. If not, the bug will devour the homeowner.
You can give this to your enemies, they neglect to feed the worm, and they get eaten. It is a very indirect form of murder.
To dispell a kodoku placed upon you, take the worm and its jar, the money acquired while it was in the house, put interest in gold and silver, and beside the road or at a crossroads. However, the person who takes the money and worm will in turn be cursed by the kodoku.
Until the bug/animal is dead or until the homeowner/target is devoured, the curse will remain.
*NOTE: It is a form of black magic, and should not be used. It is used to call death onto a person. By performing this curse, you instantly become a murderer. You have been warned.*