The core supernatural element - lingering thoughts. The unfulfilled strong obsessions of the deceased (such as unspoken last words, unfound objects, unresolved resentments) will condense into "remaining thoughts" near the places (or objects related to obsessions) that they often visited during their lifetime. Remaining thoughts are not physical ghosts, but a combination of "memory fragments + emotional energy". They cannot directly hurt people, but they will affect the memories and emotions of living people: at least it can make the person in contact repeatedly see fragments of the deceased's memory, and at worst, it can confuse the living person's own memory and make him fall into the obsession loop of the deceased.
