All contact with the spirit world is expressly forbidden irrespective of the nature of the spirits concerned (Lv 19:26-31; Dt 18:10-11; Job 7:7-10; Is 8:18-20; Lk 16:19-31). That's a problem if Saul thinks that he is doing the right thing!
1 Samuel 28:7
ESV - 7 Then Saul said to his servants, "Seek out for me a woman who is a medium, that I may go to her and inquire of her." And his servants said to him, "Behold, there is a medium at En-dor.
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Previous related eBible discussions on this subject:
https://ebible.com/questions/2364-did-the-witch-of-endor-really-summon-samuel-from-the-dead
https://ebible.com/questions/21453-why-did-god-allow-witchcraft-to-seem-real-by-allowing-samuel-to-rise-in-1-samuel-28