I would assume that there is a method to remove a spell from a spellbook.  This is pure speculation, of course.  It also may be possible to simply change the mana cost to some astronomical number that cannot be reached.

Ok, well how about being able to destroy an enemy city? I mean, in theory, I can think of how to do it to a friendly city, but that doesn't make any sense hehe.

However, i DO like the cooldown time. You can also just create a prereq for the spell so only a certain race can cast it. At least I imagine you can apply a prereq to it.

Flood and Earthquake both destroy an enemy city pretty effectively, in separate but equally entertaining methods.  I went on a small rampage and annihilated 5 towns with 5 casts  A similar effect may be achievable via any map-mod effect (perhaps even putting the map back the way you found it).

That just mean lol

It's really for a mod im working on. It would only be available to one of the races (although another one would probably have something similar). And it would take a long time to acquire, and cost a huge amount of mana, and have a really long cooldown time. That way, by the time you have the spell, people will already have a few cities (unless they are absolutely crawling on growing, which means you're done anyways).

It's still in.  Here's a lovely picture of a volcano parked on what used to be the capital of Gilden: https://forums.elementalgame.com/394500

Seems a bit overpowered as-is, but it's at least not bland