I will say this: when Demonaica suddenly busted out the spell on the tiles I was on, it was a real shock despite the fact that I knew he could do that sort of thing. It wasn't all that pleasant of a surprise, really. Before that I'd been warned when he was coming out of his keep, and I'd been able to keep tabs on his movements across the map. But I hadn't had the same sort of warning about when a spell was impending.
I think that maybe some text on him that says something like "Spell Incoming Next Turn" would be good, right there on the map. And then if you go to the region he is on, it shows you the countdown of number of turns until he will cast a spell.
Knowing specifically what he will do is bad, I agree. But knowing when he can do something, since it's predictable, is in my opinion good. Otherwise you're just requiring me to remember the invisible rules and keep track of it in my head. If I go away from a game and come back, I have to remember how many turns it has been since his last spell, etc. I think that the game should keep track of that for the player and keep them informed, while not specifying at all what KIND of spell is going to be upcoming. That would have made my experience with him in the early game a lot more pleasant; the sudden meteor storm would have been a surprise still, but of the "oh so THAT'S what an 'incoming spell next turn' meant" sort of way.
