613 Mitzvot
Negative Mitzvot 36
Not to consult the Ov

Devarim (Deuteronomy) 18:10-11
Lo-yimatse vecha, ma'avir beno-uvito ba'esh, kosem ksamim, me'onen umenachesh umchashef, vechover chaver, vesho'el ov veyid'oni, vedoresh el-hametim.
There should not be found among you, who makes his son or daughter pass through the fire, or divine divinations, or observes times, or observes signs and omens or practices witchcraft, or use incantations, or inquire of familiar spirits, soothsayer, or those inquiring of the dead.

Negative Mitzvah 8 prohibits the practice of Ov, communicating with the spirits of the dead. This mitzvah specifically prohibits consulting someone who practices Ov. Even if we do not personally practice this form of divination, we should not go to see those who do for direction.

This type of necromancy often involves the use of a human skull. It might involve ventriloquism, where the medium makes a voice appear to come from under his arm (Sanhedrin 65a,b) or from the ground.

Yeshayhu (Isaiah) 29:4
You will be brought down, and will speak out of the ground. Your speech will mumble out of the dust. Your voice will be as of one who has a familiar spirit, out of the ground, and your speech will whisper out of the dust.

Bible.Ort.org also says concerning this practice: "The methods can also involve meditation and incense drugs (Yad, Avodath Kokhavim 6:1; Sefer HaMitzvoth, Negative 2). He can also use these means to produce illusions and hallucinations (Saadia)."

1 Divre HaYamim (Chronicles) 10:13
So Sha’ul died for his trespass which he committed against YHVH, because of the word of YHVH, which he didn’t keep; and also because he asked counsel of one who had a familiar spirit, to inquire thereby.