Saul had killed the priesthood, with the last priest going off to be with David, so the Urim he consults will not work, showing even moreso that Saul does not understand what God desires.
Saul shows his true colors, as he quickly moves on to the help of a medium to receive his answer.
Saul disguises himself and goes by night to the medium at Endor (vv. 7–8).
He wrongfully swears by the LORD's life that no punishment will come upon her (v. 10).
Samuel appears (TODO: See what Conley wants you to do here) (vv. 12–14).
The plain reading of the text seems to be that this is Samuel himself, and that God has ordained such a thing to occur.
Necromancers have no true power to retrieve dead men from the afterlife, but God has all means at his disposal to do His will, and in this case, used to rebuke Saul in a distinctly powerful manner.
Samuel's message confirms again that the Lord is serving judgment on him, and clarifies to Saul that he will die, unrepentant, at the hands of Israel's enemies.
Saul's kingship will soon end.
The line of David will soon take over rule from this wicked king.