Because of wars the number of spirits increases, this has left the Spirit World no longer able to handle it. When a person reaches a certain level, the Spirit World will send a messenger to ask him to work for them. The Reward is that, when the person dies, hen can immediately go to the Spirit World. No matter how benevolent one lives their, as long as there's a trace of resentment in them, they'll be rejected by the Spirit World.
Orginally Requiem Street and the Spirit World used to be one, united as as the Spirit World. Due to the never ending chaos, the number of evil spirits increased rapidly over time. To average departed spirits with the strength of a newborn kitten, it was a disaster. More and more innocent spirits became the subject of toture for the evil spirits. With the help of the power the Divine Pearl of Xihe, the Spirit World was divided into two grounds, Requiem Street and the Spirit Worlds. Breaking the laws of the Spirit World, the Spirit defenders invited humans into Requiem Street. The Humans were then given the responsiblity of supervising the Spirit World.