King Saul was about to enter a war with the Philistines.  Saul used to consult Samuel in times of desperation, but Samuel had died.  Saul was not hearing the voice of the Lord, and desperately wanted to hear it.  Saul went to the Witch of Endor to have her bring up the spirit of Samuel to speak to Saul.  This was evil and against God's laws.  Samuel prophesied that King Saul and his sons would die in battle against the Philistines the next day.  It happened just as Samuel said.