Yeah I've had the AI start the Spell of Making on me about four times. I've seen it complete it at least once.
I lost when it completed the spell, although it was a bit buggy, I think the end screen said I'd won a Conquest victory or something.
In my current game the AI is casting the Spell of Making, I should be able to stop it by capturing its Towers (although it's highly unclear from the documentation exactly how you do stop the Spell of Making). The last time I stopped it casting the Spell of Making I killed its sovereign, nothing happened. I took the cities with its towers in, nothing happened. Eventually it completed casting the Spell of Making and I got a highly flavoursome message something along the lines of "Casting failed- does not meet requirements". So I had stopped it, but I've no idea which step I took stopped it, you only find out at the end of the ten turns whether the AI has won or not.
This may be why an AI seemed to complete the Spell of Making in the Insane game I actually won but nothing happened and then it immediately restarted; it may be that one of the other AIs had taken a city with one of the towers in but the Spell only actually failed when it got to the end of the ten turns. This is speculation though, I got no feedback at all. (More feedback from the game as to why the Spell failed would be nice!)
Reading the in-game help, the only clear requirement other than mana etc. is that you have the Forge of the Overlord. So I assume capturing the Forge of the Overlord will stop it. I'm not sure if capturing any of the other Towers will stop it.
I invariably play on Ridiculous or higher, I've never actually seen it complete the Spell of Making on Ridiculous. I don't think I've seen it start the Spell of Making on Ridiculous. I'm fairly sure I've seen it complete all three towers and the Forge of the Overlord, but it never started the Spell of Making.
As I say I've seen it start casting several times on Insane. I'm currently playing on Ridiculous+ (Primal Savage's Xtra Savage mod), and Magnar is currently casting the Spell of Making, but I should be able to stop him by capturing his towers and the Forge.
I assume the difference between being able to start the Spell of Making on Insane vs. lower difficulty levels is simply a matter of resources. I think the biggest problem normally is the AI saving up enough mana, as normally by that stage it's at war with multiple factions and is using up its mana from turn to turn. On Insane it gets such crazy bonuses it can build up the necessary mana without really trying.
