Oyashiro-sama

Oyashiro-sama is the shrine god in the town of Hinamizawa. He is depicted as a very wrathful deity that would be quick to punish anyone who displeasures him in any way. Throughout the series, characters have been shown going insane, and it was believed that Oyashiro-sama was putting a curse on these villagers whenever they thought about leaving the village. However, in Kai, it's revealed that the characters were going insane because of a disease called the Hinamizawa Syndrome, and that Oyashiro-sama was a real deity, but instead of being a male like he's presented in the mythos, he's actually a purple-haired spirit girl named Hanyuu Furude. She was horrified when she discovered that her name was being connected to a god that was committing very terrible crimes against the villagers.

Oyashiro's Curse
According to legend, if a villager ever thought about leaving Hinamizawa, Oyashiro-sama would strike that person down with madness. In order to escape this punishment, it was recommended that one should do each of the following:
 * 1) Never talk about Oyashiro-sama.
 * 2) Never enter into the village.
 * 3) Never leave the village.
 * 4) Do not even think about leaving the village.
 * 5) Never defy Oyashiro in anyway.
 * 6) Never enter into the Saiguden without permission. Only the priestess and her family can come in anytime they wanted.
 * 7) Apparently, even the Furude family is liable to the curse.

Trivia

 * He's very similar to the Maltheistic version of God.