What is "gwisin"

Gwisin is a general term for ghosts in Korea. They are considered similar to a Yogoe ; they are people who have died, not monsters or creatures such as Dokkaebi. They can be found in many places, but most commonly in abandoned buildings, houses, cemeteries, forests, etc. When a person dies without completing something that they must do (e.g. revenge or staying longer with their family), their spirit remains on earth to complete the task before going on to the underworld. However, very strong gwisin do not go to the underground world if they want to remain on earth, and over time these gain strength.