In ghost hunting, a "simulacrum" is a term often used to refer to an entity that is the likeness of someone or something once living but may not be their actual ghost or spirit.
Certain religions, especially those that do not hold beliefs in the existence of ghosts, will often teach that ghosts are not disembodied people but devils or demons that deceive the unsuspecting into believing they are the spirits of humans.
Upon witnessing a ghost, such as in the case of the very well-documented Nelly Butler, unbelievers were afraid and thought the devil was out to deceive them:
"Even when they saw the phenomenon of her appearance, or heard her voice, some of the people of the community did not believe they were witnessing the actual ghost of Mrs. Butler. They had recourse to only one other theory to explain the apparition, but it seemed logical to them. They believed that this wraith of a dead woman must be a 'simulacrum fashioned by the Devil.' This was the position taken by Nelly's sister, Mrs. Sally Wentworth, although her parents were quite convinced to the contrary."
- excerpted from the book, "Prominent American Ghosts" by author Susy Smith
Ghosts That Can Alter Their Appearance
In cases of haunting that seem to involve wicked ghosts, witnesses will often describe seeing entities that appear as something frightening to them, such as a horned devil, or a very tall figure having red eyes.
This type of ghost manifestation may also be defined as a simulcrum -- as some ghost investigators have found that ghosts who have been earthbound long enough may learn to alter their appearance in order to frighten unsuspecting victims. Invoking fear in such a way can give villanous ghosts an upper hand over the living, allowing them to control people.
Through communication and working through such cases, we had found some of the simulacra to actually be ghosts of people who could transition to a higher estate with loved ones in the spirit realm. These loved ones (spirits of family and friends), it would seem, were always attempting to reach the depraved souls but were often ignored. Good ghost investigation teams capable of bridging the gap between worlds can make a difference, helping all tortured souls find peace and, essentially, clearing the haunt.