This is my theory, and also my personal hope, because I love Vez'nan, he was my favorite hero in origins and my all time favorite boss. My theory is this: It's possible Vez'nan never recovered his staff, and thus, the effects of the corruption wore off. The end of the game would be that: Vez'nan and the general (and possibly...the king?!) would meet again in battle, and Vez'nan would be there, mocking you as usual, but at the end, instead of him being the boss, a greater evil shows up, and Vez'nan is forced to join forces with the king to fight it.