A familiar spirit is a demon that is summoned by a medium. I carm.org

What is a familiar spirit?

by Matt Slick | Jan 24, 2014 | Demons, Questions

A familiar spirit is a demon that is summoned by a medium with the intention that the spirit summoned will obey his or her commands. Often, familiar spirits are believed to be the spirits of people who have died (Duet. 18:11). However, biblically this is not the case. Such appearances are in actuality demonic forces imitating people in order to deceive.

We know that people cannot come back from the dead from what Jesus said in Luke 16:19-31 when he gave us the story of Lazarus and the rich man. Jesus said in verse 26 “and besides all this, between us and you there is a great chasm fixed, so that those who wish to come over from here to you will not be able, and that none may cross over from there to us.” So, we can see that people cannot come back from the dead. However, we cannot leave this topic without addressing 1 Samuel 28 that deals with Saul and the medium who, apparently, actually contacted the departed Spirit of Samuel. Commentators are split on whether or not this was actually Samuel, but the text seems to imply it was. So why the exception? We are not sure, but it might have been allowed for the purpose of rebuking Saul for his sin, something that does not happen with familiar spirits when they manifest. Perhaps it is a teaching tool to show us the difference between those who are really of God (Samuel) and those who are not (familiar spirits).

The two words “familiar spirit” in the King James Bible are translated into English from the single Hebrew word obe אֹוב.

King James Bible

New American Standard Bible

Lev. 19:31, “Regard not them that have familiar spirits, neither seek after wizards, to be defiled by them: I am the LORD your God.”
1 Chron. 10:13, “So Saul died for his transgression which he committed against the LORD, even against the word of the LORD, which he kept not, and also for asking counsel of one that had a familiar spirit, to inquire of it.”
2 Chron. 33:6, “And he caused his children to pass through the fire in the valley of the son of Hinnom: also he observed times, and used enchantments, and used witchcraft, and dealt with a familiar spirit, and with wizards: he wrought much evil in the sight of the LORD, to provoke him to anger.”

Lev. 19:31, “Do not turn to mediums or spiritists; do not seek them out to be defiled by them. I am the Lord your God.”
1 Chron. 10:13, “So Saul died for his trespass which he committed against the Lord, because of the word of the Lord which he did not keep; and also because he asked counsel of a medium, making inquiry of it.”
2 Chron. 33:6, ” He made his sons pass through the fire in the valley of Ben-Hinnom; and he practiced witchcraft, used divination, practiced sorcery and dealt with mediums and spiritists. He did much evil in the sight of the Lord, provoking Him to anger.”