The internal evidence from the narrative reveals a number of significant details to conclude that the spirit the medium saw was not Samuel but a demon impersonating him. One of the most obvious clues is that the text tells the reader outright—twice!—that the Lord would not answer Saul (I Samuel 28:6, 15-16), and there is no way that God would answer him through a lying spirit during an abominable séance! One of the points of the story is to show what desperate people will do when they are cut off from God, in fear for their lives, and without hope.
Yet, this does not mean that the demon does not give Saul a truthful answer. Acting as if it were Samuel, the demon wounds the king with the cruelest words it can use, complaining about being disturbed in his rest, mocking Saul for seeking him, and rubbing it in that God had left him and become his enemy. It reminds him of one of Samuel's prophecies—given when Saul had disobeyed God's instruction about the punishment of Amalek and its king, Agag (see I Samuel 15)—foretelling that the kingdom would be torn from him and given to another, David (I Samuel 15:23, 26-28). Finally, it predicts that both he and his sons would die in the next day's battle against the Philistines, a reasonable assumption considering how overmatched Saul's forces were.
The demon's words have the desired effect: “Immediately, Saul fell full length on the ground, and was dreadfully afraid because of the words of Samuel. And there was no strength in him, for he had eaten no food all day or all night” (I Samuel 28:20). Playing on Saul's fears and weakness, the demon succeeds in bringing the big man low, destroying any remnant of hope. Later, after finally eating and resting (verses 21-25), he leaves the medium's house a completely broken man.
So, what happened at En Dor?
1. At the end of his rope and highly susceptible to suggestion, Saul was ready to clutch at any straw of hope for a better outcome.
2. The medium was a fraud, bilking people of their money by preying on their superstitions. The spirit's appearance shocked her.
3. At most God allowed a demon to impersonate Samuel and pronounce Saul's doom to him, to give him the truth from the only source he had ever trusted to speak straight to him.
In the end, the story of Saul and the medium at En Dor is a morality play of sorts, an object lesson to teach how dangerous it is to forsake God and turn to the counsels of demons through sorcery and divination. It is a path of fear, despair, lies, curses, and death. It records the sad and tragic end of a man who had shown such great potential but who had allowed jealousy and pride to bring him and his house to ruin.