In Game of Thrones, there are many cultural practices and religions all across Westeros. One of the oldest religious practices is a belief in the Old Gods. These beings were worshipped by the Children of the Forest before men began practicing it. Specifically, the only region with Old Gods primarily worshipped in the northern region of Westeros. The basis for this religion was animism, Wicca, Norse mythology, and Celtic rituals according to George R.R. Martin.
While the Old Gods were considered omniscient and connected to nature, they weren’t formally named. Many believed they watched through the trees and that the power of the Old Gods was directly tied to heart trees. Heart trees are Weirwood trees with faces carved into them, creating an idol to the gods in a way.
When an area of heart trees was destroyed, it was said that Old Gods no longer had power in that area. Certain groups of man worshipped gods directly correlating with a certain piece of nature. There were dark gods underground, gods of ice and snow, crab gods, and more. Certain practices such as the sacred law of hospitality are sacred to the degree that legal laws respect it. If someone was to break this law, it was believed they would suffer the wrath of the Old Gods.
When the Andals invaded Westeros, they brought their own religious practice commonly called the Seven. This led to why the majority of Westeros besides those from the North did not worship the Old Gods. The Old Gods are worshipped by those beyond the wall, which fits with how most Northerners still practice the religion.