What prompted Sha'ul to ask from a Ba'alas Ov? This is one of the two reasons why he died (Divrei ha'Yamim 1 10:13)!
Malbim (4-6): (a) Shmuel died (and Sha'ul was afraid). (b) Pelishtim gathered and camped in Shunam, in Eretz Yisrael. Sha'ul feared to approach them; he camped in Gilbo'a. (c) Sha'ul understood that his fear is like a Nevu'ah for evil. (d) He asked from Hashem, and was not answered.
Why did he request specifically a female Ba'alas Ov?
Radak: This is more common among women than men, for Daitan Kalos. 1 This is why it says (Shemos 22:17) "Mechashefah Lo Sechayeh" (a female witch).
I.e. they easily succumb to temptation. Or, they do not concentrate so well, so it is harder from them to reach Ru'ach ha'Kodesh. Therefore, they seek other ways to know hidden matters. (PF)
Why did he say "v'Edreshah Bah"?
Kli Yakar #1: She was a master of it, i.e. she knew how to begin arousal, without completing the Aveirah of asking from the Mes. Sha'ul needed to go himself, so she will do only the permitted initial stage, and through her act, he will be able [to get an answer from Shmuel]. This is why Avner was willing to say that his mother was a Ba'alas Ov. Really, even part of the process is forbidden (Divrei ha'Yamim 1 10:13 calls it an Aveirah for Sha'ul. - PF)
Kli Yakar #2: I will see if she is truly a Chacham about this, or does folly - if she can tell who I am, and whom I want to bring up, without hearing from me. Refer to 28:8:4:2, 28:8:5:1.
Malbim: Initially, he did not intend to ask from the Mes; he would ask her, and she would ask the Mes. He thought that the Torah forbids only the one who asks from the Mes. 1 When Shmuel directly addressed Sha'ul, he himself asked.
In any case, since she asks due to him, he transgresses "v'Lifnei Iver" (especially since she herself feared to do so)! In any case, this is a smaller Aveirah than asking from the Mes. (PF)
Who was this Ba'alas Ov?
Radak (from Pirkei d'R. Eliezer 32 or 33): She was Tzefanyah's wife, the mother of Avner. 1
Seforno (Devarim 18:10): She was a Nochris. 2
V'Lo Od Ela (33): He eradicated only those who used the Chachmah to conjure up Mesim. Refer to 28:7:5:1.
Granted, she would fear Sha'ul, for Seforno explains that we are commanded to eradicate also Nochri Ba'alei Ov. However, why would a Nochris risk her life for Yehudim? And why was she so concerned that Sha'ul eat and have strength? And how could Sha'ul and his men eat from what she slaughtered (verses 24,25)?! Perhaps Seforno explains "va'Tizbechehu" (24), i.e. she told one of Sha'ul's men to slaughter it. (PF)
Pirkei d'R. Eliezer (33) says that the Ba'alas Ov was Tzefanyah's wife, the mother of Avner. Was she a Rasha?!
V'Lo Od Ela (33), Chomas Anach, both citing Kli Yakar: She learned like the Sanhedrin learned [witchcraft - in order to be able to rule about it], not in order to do it.
