Translated from original post:

<Lamia girl/serpent goddess>
In a certain village, a family of Lamias protect the village and their leader is worshipped as the serpent god.
In ages past, when humans and demons were at war, some unusual Lamia girls fought on the side of the humans and protected them, even today in this age of peace, their descendents continue to carry that role as symbolic protectors of the village.
When the serpent god's daughter reaches adulthood, a "child" from the village is assigned to her. The role is that of a secrifice, but the child is never mistreated, rather it is given respect and affection. The child is generally chosen from around the age where they have reached or are about to reach puberty.
The boys of the village have had many opportunities from a very young age to visit the shrine where the Lamia girls live for various ceremonies and festivals. During that time, the daughter secretly selects a boy that is to her liking...
In some cases, to test how comfortable the boy is to hold, the Lamia girls will take turns wrapping themselves around him and gently embracing him. Some of the boys experience their first love or the awakening of their sexuality here.
Those chosen will live at the shrine from then on. They are given daytime chores, but fulfilling their "nighttime duties" is considered of utmost importance.
In other words, his duty as a source of magical energy, sexual pleasure, and as a breeding partner for the Lamia girl.