Are ghosts biblical? 

Yes. King Saul consults a lady (medium) at En Dor to raise the prophet Samuel from the dead. In verse 12 she looks surprised to see Samuel, which leads us to believe God brought the ghost of Samuel to earth apart from her doing so. This was a unique  case that God allowed for his own divine purpose. People tend to have active imaginations and see things that are not really there. 

1 Samuel 28:7-15

Then Saul said to his servants, “Find me a woman who is a medium, that I may go to her and inquire of her.” And his servants said to him, “In fact, there is a woman who is a medium at En Dor.” So Saul disguised himself and put on other clothes, and he went, and two men with him; and they came to the woman by night. And he said, “Please conduct a séance for me, and bring up for me the one I shall name to you.” Then the woman said to him, “Look, you know what Saul has done, how he has cut off the mediums and the spiritists from the land. Why then do you lay a snare for my life, to cause me to die?” And Saul swore to her by the Lord, saying, “As the Lord lives, no punishment shall come upon you for this thing.” Then the woman said, “Whom shall I bring up for you?” And he said, “Bring up Samuel for me.” When the woman saw Samuel, she cried out with a loud voice. And the woman spoke to Saul, saying, “Why have you deceived me? For you are Saul!” And the king said to her, “Do not be afraid. What did you see?” And the woman said to Saul, “I saw a spirit ascending out of the earth.” So he said to her, “What is his form?” And she said, “An old man is coming up, and he is covered with a mantle.” And Saul perceived that it was Samuel, and he stooped with his face to the ground and bowed down. Now Samuel said to Saul, “Why have you disturbed me by bringing me up?”

Video - What does the Bible say about ghosts & hauntings? - Got Questions . Org