Want to have some fun, make it LOOK like the mage broke out and is now harassing the PCs. In fact, could be an associate, a copycat, or to make it personal; a family member. Heck, why not more than one person duing it for additional screwing-with. Illusion spells are a wonderful thing. Heck, it dosen't have to be that elaborite, mess with them by having Evil Mage 2.0(s) have the EXACT M.O., just for added creep factor and play it up so that the PCs THINK something is going on, only to keep going back to find Evil Mage Prime still safely tucked away in his cell, a la the movie "Law Abiding Citizen".