1 Samuel 28:7 Saul instructed his servants, “Find me a woman who is a medium, so that I may go to her and inquire of her.” His servants replied to him, “There is a woman who is a medium in Endor.”
28:8 So Saul disguised himself and put on other clothing and left, accompanied by two of his men. They came to the woman at night and said, “Use your ritual pit to conjure up for me the one I tell you.”
28:9 But the woman said to him, “Look, you are aware of what Saul has done; he has removed the mediums and magicians from the land! Why are you trapping me so you can put me to death?” 28:10 But Saul swore an oath to her by the LORD, “As surely as the LORD lives, you will not incur guilt in this matter!” 28:11 The woman replied, “Who is it that I should bring up for you?” He said, “Bring up for me Samuel.”
28:12 When the woman saw Samuel, she cried out loudly. The woman said to Saul, “Why have you deceived me? You are Saul!” 28:13 The king said to her, “Don’t be afraid! What have you seen?” The woman replied to Saul, “I have seen one like a god coming up from the ground!” 28:14 He said to her, “What about his appearance?” She said, “An old man is coming up! He is wrapped in a robe!”
Then Saul realized it was Samuel, and he bowed his face toward the ground and kneeled down. 28:15 Samuel said to Saul, “Why have you disturbed me by bringing me up?”

Did Samuel actually appear, or just some evil spirit pretending to be Samuel?

Some, including Payne and Baldwin, express agnosticism on this question.  So also Evans: ‘we have no way of knowing whether this was a trick, the appearance of some kind of evil spirit or an actual visit from the dead Samuel.’ (DOT:HB).  Youngblood also:

‘My own sympathies lie with the judgment of Gregory of Nazianzus, who was content to leave the text in its ambiguity: “Samuel was raised, or so it seems by the woman having a familiar spirit”.’ (Emphasis not in original)

If we are to go further, and seek a definite answer to the question, we must consider the following possibilities:

1. A deception on the part of the woman
2. An illusion or hallucination
3. An actual appearance of a demon caused by the woman
4. An actual appearance of a demon caused by God
5. A demonic deception.  Youngblood quotes the Geneva Bible: ‘“was Satan, who to blinde his [Saul’s] eyes tooke upon him the forme of Samuel, as he can doe of an Angel of light” [alluding to 2 Co 11:14]).’  This view is also taken by Matthew Henry, Matthew Poole, and others.  Henry: ‘That the devil, by the divine permission, should be able to personate Samuel is not strange, since he can transform himself into an angel of light! nor is it strange that he should be permitted to do it upon this occasion, that Saul might be driven to despair, by enquiring of the devil, since he would not, in a right manner, enquire of the Lord, by which he might have had comfort.’

Youngblood explores some of these less-than-satisfactory options:

‘Early church fathers, fearful of affirming that the prophet Samuel was a shade in Sheol, that a medium was an appropriate intermediary between the divine and human worlds, and that necromancy is efficacious, “proceeded to undermine the literal text with one of two arguments: either sorcery is just demonic deceit, and what appeared was not really Samuel, but a demon in his guise; or, Samuel was not really in Hades but had been sent by God to announce Saul’s fate”’ (Quoting Cox)

With regard to 1 and 2, ‘we must remember that Scripture describes such practices not as futile but as pagan. Yahweh forbids Israel to use these means not because they do not work but because they are wicked.’ (Davis)

Interpretations 3 and 4 are also unsatisfactory.  This does appear to have actually been a post-mortem appearance of Samuel.  Both Saul and the narrator identify the figure as ‘Samuel’.  This figure speaks directly to Saul (not through the medium).  Furthermore, the message spoken is faithful to the real Samuel.  There is no hint of demonic deception in it.  Indeed, the message contains allusions to Samuel’s previous encounter with Saul.

So, we are left with one of the following:

1. An actual appearance of Samuel by the means of necromancy.   The OT condemns necromancy (Lev 19:31; Deut 18:9–14), but does not say that it is absolutely impossible.  As Bergen says, ‘the Torah prohibits necromancy not because it is a hoax but because it promotes reliance on supernatural guidance from some source other than the Lord.’

The woman’s reaction may suggest that, to her horror, she had indeed brought up Samuel himself.  ‘Her strong reaction also suggests that Samuel’s appearance was unexpected; perhaps this was the first time she had ever actually succeeded in contacting the dead.’

Samuel’s question, “Why have you disturbed me by bringing me up?” suggests that he was sent reluctantly

If a godly man such as Samuel could be summoned (presumably unwillingly) from the dead through occult practices, then this would raise serious theological problems and leave us wondering if we ourselves might be similarly manipulated by evil powers and not permitted to rest in peace.  To this prospect, Gerald Bray responds with an emphatic ‘No’ (God Is Love: A Biblical and Systematic Theology, p368).

2. An actual appearance of Samuel caused by God.  In this case, ‘it was not the skill of the medium but rather a unique act of God that brought Saul into contact with Samuel. The medium did not possess the capacity to disturb a dead saint; but God, as “a sign of his grace,” permitted Saul to have one last encounter with the prophet who had played such a determinative role in the king’s career’ (Bergen).

It was, then, an exceptional occurrence, analogous in a way to God’s use of astrologers to herald the glad tidings of the birth of his Son.

Woodhouse perceptively remarks: ‘It seems [very] likely to me that the Lord sent Samuel to Saul on this evening, just as on a very different occasion he sent Moses and Elijah to Jesus (Matthew 17:3).’

This last appears to be the best interpretation.  It is supported by most conservative commentators.