A revenant is the shadowy reappearance of a deceased person in the place where he lived.
Dr. Kurt Koch writes in his book „Occult ABC“:

Not all recurrent experiences can be reduced to a single denominator. Some forms are mentioned.
1. There are reports of people of faith seeing their deceased mother, who successfully warned them of the danger when their lives were in great danger.
2. There are also many examples of ancient figures appearing in old houses. The so-called „Hambach Ghost“ was observed for three hundred years until a cistern containing the skeletons of children and women was discovered during the demolition of the house. It was said that the cistern had belonged to a monastery centuries ago.
3. Revenants can also be materialised demonic apparitions. Luther pointed out that demons and evil spirits take on the appearance of the dead in order to seduce the living.
4. Revenants can also be the product of a morbid imagination or symptoms of mental illness.
5. Revenants can be projections of the imagination of healthy people with an eidetic disposition.

Kurt E. Koch gives the pastoral advice not to get involved with revenants. Only in case 1 is it good to check the advice given. This can then be accepted or rejected. For there is also pious deceit.

Kurt E. Koch: There are recurrent experiences that can be understood as external projections of the human subconscious. For example, if a young woman has lost her husband in a car accident, she might one day see him again, either in a dream or half asleep. This is simply an external projection of their desires. In addition to these „immanent“ revenants, which can be explained by the deep layers of the human soul, there are also true revenants.
We have such reports from ancient times. However, here are examples from the present:

B 364 A pastor’s wife in France reported a revenant in her grandparents‘ house. A revenant was observed in the house for decades, according to chronicles even for several centuries. The house was built in the 13th century. There used to be a café in the building called „Tannenzapfen“ (Pine cone). Due to its dilapidated state, the house was demolished a few years ago. In the foundation walls a walled in skeleton was found. So there was probably a crime here. Once again, we have the theory that houses where crimes have been committed are sometimes haunted and unsettle the occupants.

B 368 A deaconess was troubled for a long time by the fact that she saw a hideous-looking revenant at night. These apparitions led her to the pastoral care of a pastor who interceded on her behalf. The pastor had never had a case like this in his pastoral care, so he was unable to give the deaconess proper advice. Moreover, from that time on, the pastor himself was subjected to severe trials when he prayed for the sister. He felt physically attacked each time and also felt a tight grip on his throat when he tried to pray for the sister. After these challenges, I was consulted in the pastoral care of this sister.
There are also revenant experiences among people who are psychic or burdened by witchcraft sins. This explains why the priest was strangled and physically attacked as he prayed for the sister. The sister was undoubtedly occultly afflicted. And revenants prefer to show themselves to people with a certain level of psychic ability. At times, it seems as if the revenants from the afterlife are using the psychic power of a person with a psychic burden to make themselves visible. There are processes of this kind in the spiritistic sessions.

The warning should be repeated that one should not deal with revenants, i.e. not ask them any questions and not allow them to appear in the house, even in the name of Jesus. We have the protection of the living God and need not fear these machinations of darkness. (Source: „Occult ABC“, Dr. Kurt E. Koch)