Those who attempt to speak to the dead are dabbling with the demonic realm, a forbidden occult practice that is punishable by stoning: “A man or a woman who is a medium, or who has familiar spirits, shall surely be put to death; they shall stone them with stones” (Lev. 20:27).
Unger’s Bible Dictionary tells us: “[A] familiar spirit is a divining demon present in the physical body of the conjurer…The term ‘familiar’ is used to describe the foreboding demon because it was regarded by the English translators as a secret (famulus), belonging to the family (familiaris), who was on intimate terms with and might be readily summoned by the one possessing it.” God prohibits “one who practices witchcraft, or a soothsayer, or one who interprets omens, or a sorcerer, or one who conjures spells, or a medium, or a spiritist, or one who calls up the dead. For all who do these things are an abomination to the LORD” (Deut. 18:10–12). God killed King Saul after he sought guidance through a familiar spirit, rather than through the Lord (see 1 Sam. 28:7). Those who desire the supernatural should seek God through prayer and through His Word.
1 Chronicles 10:13 So Saul died for his unfaithfulness which he had committed against the Lord, because he did not keep the word of the Lord, and also because he consulted a medium for guidance. 14But he did not inquire of the Lord; therefore He killed him, and turned the kingdom over to David the son of Jesse.