I just managed to complete it yesterday. Every other time something happened with my saved games or disks and I wasn't able to see the ending. Come to find out, there are lots of endings, but apparently you really need to be a mage to see the best ones. I may try to redo the last level after stocking up on cranium rats just to see what the sphere does.
In retrospect, it's probably best to play as a mage or at least a fighter with a very high int.