The Bone Mound was located to the north of the Ash Valley Forest, which was actually a Natural Grand Canyon. However, due to its unique geographical environment, it hardly ever saw sunlight all year round and had a complex terrain, resembling a gigantic labyrinth.
Some people, wild beasts, or even certain demons who accidentally fell into the Bone Mound, if unaware of how to leave, would be unable to escape once they fell in, and would ultimately become just another skeleton within the Bone Mound.
After a long period of accumulation, the number of skeletons inside the canyon grew, and so did the resentment, pain, despair, and other negative emotions of the tens of thousands of creatures before their death. These emotions had accumulated over the years inside the canyon, resulting in an exceptionally high concentration of negative energy particles within the Bone Mound.