The word 'yulyeong' is synonymous with dangerous ghosts, unlike gwishin which can vary in danger levels and temperament. That makes it all the more ominous and appropriate that this type of ghost is what lends its name to the common nickname of Keobeu Pass.
The land known as Keobeu Pass is high in the mountains between Unknown and Unknown. Primarily known in history as the site of The Battle of Keobeu Pass, the area is considered cursed land and is known to be riddled with not only undead but also with mass graves from the dead and the remnants of ancient curses from the Warring Houses period. Despite generations of effort, The Imperial Gwishin Bureau has been unable to relieve the area of this resentment and oppression. It has sunk so deeply into the soil that it even effects the plants and animals in the area.