Our reality is separated from the realm of chaos by a thin barrier that prevents either realm's inhabitants from interacting. The exception is during an event which occurs every 1000 years. In this scenario where the sun, earth, and moon align, it causes a solar eclipse, which opens a path from the chaos realm into our world. Think of a balloon which has a hole, allowing outside air to come in and deflate it. The barrier is bypassed, allowing demons to flood into our reality. Fortunately, this event is temporary, and demons lose power closer to the end of the eclipse. When the event is over, the energy sustaining the demons dissipates, and the spirits fade back into their realm.
After the devastation, the world does what it can to go back to normal. However, some of the energy from the chaos realm coalesces in certain areas. This is the generation of certain "haunted" locations, such as houses, caves, etc. The evil spirits are sustained by the power of the remaining energy, harming or killing anyone who trespasses.
However, there is a flaw in this logic. After a rip in a balloon is repaired, it isn't fixed completely. Even a small tear will allow air to penetrate, which will eventually cause it to deflate. At best, you have only bought time. These trapped energies in various locations should be spreading to the rest of reality. Instead, they are trapped in these locations, frustrating the spirits within as it prevents them from leaving. Why would this be the case?