According to the Torah (Vayikra 20:27) someone who engages in these activities is liable to receive a death penalty; someone who approaches an ov or a yidoni for the purpose of engaging their services is liable to receive lashes (see Vayikra 19:31). The Mishna teaches that a ba’al ov is someone who presents himself as able to tell the future by means of his wizardry. The Rambam describes his activity as follows: the ba’al ov (necromancer) burns incense and holds a myrtle branch in his hands, waving it and reciting a magic formula until the questioner hears what appears to him to be an answer to his request coming from the depths of the ground. Similarly, someone who takes a skull and holds it in a manner that makes it seem as though a voice was coming out from his body. A yidoni (sorcerer) is someone who takes a bone of an animal or a bird and places it in his mouth so that it appears that a voice speaks of the future or of magical things.
One case of a ba’al ov that is mentioned by the Gemara is described in Tanakh where King Sha’ul – who had eradicated the witches from Israel –searches out an eshet ba’alat ov in order to call up the prophet Shmuel from the dead in order to get his advice (see I Shmuel chapter 28) before a war with the Pelishtim. According to the Gemara, Shmuel did not appear in an ordinary fashion, but spoke through the body of the eshet ba’alat ov.
While the description in Sefer Shmuel makes it appear as though the eshet ba’alat ov succeeded in actually calling Shmuel from the dead, the Ge’onim suggest that this is a combination of sleight of hand and psychology. The eshet ba’alat ov was trained to understand the personal needs of the clients who approached her. She feigned surprise at the discovery that the man standing before her was King Sha’ul, but the message she delivered – that he would be killed in battle – was not from the prophet, Shmuel, but was her own.