Evil wizard from 4000 years ago. They manage to catch and stop him, but they can't kill him because he has Physical Immunity (stone heart). They seal him up, bound, in a cave, hoping to eventually track down his heart (he has powerful wards protecting it from divination). 50 years later, a supernatural war happens and this group of casters is killed. Now, in the year 2010, this evil wizard finally breaks his way free of confinement! Or maybe some curious heroes broke him loose by accident (dang kids).