The Skull of Pain (1997)

Mary Sotherland has escaped a deranged killer who murdered six of her friends. Now, a year later, another friend disappears and a human skull shows up on her doorstep along with a letter that reads: "You'll find her body through the skull." Mary has 24 hours to track down the buried alive body of her friend before the killer murders her. And every two hours that pass, another one of Mary's friends is killed.