A Kodoku is a mainstream curse in many different Onmyoudou styles that become curse-type shikigami using spiders or centipedes as a vessel. They have a dark presence, made of dark fog and filled with countless eyes. It is forbidden to use or create one without the express permission of the Onmyou Agency.
Kodoku are created by placing a large amount of insects in a pot-like tool and made to kill each other until the last one remaining was the strongest. This bug is used to perform the kodoku, namely as the sacrifice to serve as the vessel. By infusing it with the magical energy of the curse, that is to say the spiritual power of negative emotions and resentment, it would become a shikigami.
Kodoku are somewhat difficult to destroy due to being most gaseous shikigami, and their touch is harmful to most others. This makes them capable of causing serious damage to both shikigami and humans alike. However, running water can weaken them and counter-curses will send them back to their creator.