Well, after Ice Sword, there haven't been any mentions of Tarnum... But what if some Necromancer resurrected him? And Tarnum either remembered who he was or something else happened... :) And the idea of resurrection is good, and then continue with the Necromancers. Or maybe demons were burning everything, and he had to stop them, because demons are scary :) and the undead don't care who or when to kill. And who is friends with scripts? Maybe we can add some interesting things to the campaign (I'm a fan of Heroes, but my soul really longs for the RPG genre).
If you're going to make an RPG map, it's better to do it in Heroes 4... there are much more possibilities and various scripts...