I really like this idea. It's a good builder victory, but at the same time forces you to build your troops and prepare for a one final defensive war. Reminds me of the victory condition idea I suggested sometime for FfH I, I think - a system that gives you increasing diplomatic penalties the closer you get to finishing your goal. Research 10% of the beakers and you get "-1 you're planning a super spell to finish us off" from everybody, then ever-increasing penalties the closer you to get to completion. Finally you're at the point where just about
all the computer players will declare war on you just to prevent you from winning, while you're mounting a desperate defense to buy enough time to bring that spell to completion. Moreover, other AI players will probably be researching the spell as well, so the end game will end up as a huge all-out battle with everybody getting drawn into wars with each other.
Ahh... such a neat little apocalyptic scenario. I don't think anybody bothered to comment on it when I last suggested it, though.