A Gyochu is an extremely tiny yokai that is a type of divine-curse for those who break the sacred tradition of Koshin night by engaging in sexual intercourse.
This causes a Gyochu to infect their body, where it will forevermore reside within their reproductive organs.
Gyochu are strange six-armed monsters with long red tongues who while not dangerous in themselves do carry a malevolent presence as after infecting a host they will collect information on all the host's evil deeds or sins against the gods, leaving their bodies each Koshin night and traveling to Hell in order to tell Enma Daio, who will then bring great torment upon the host, determined by the weight of these sins.
The only way to avoid getting infected by a Gyochu is to respect the sanctity of the Koshin night and refrain from engaging in sex during that time period, which is seen as an affront to the gods.