[Tortanick]

That dosn't make any sense though for most spells.

Dark Skies. Gift of health. Fata Morgana.

Give me one reason why those spells can't be cast in seperate empires.

Other spells like Utterdark & second sun could be limited with a message like "someone else has done the hard work for you"

And there are a few spells that couldn't really be cast more than once, Motheroak. Well of Missery. Arcane Nexus. For those spells I'd say you'd have to use more gems then the current one to redirect/relocate the spell. For balance reasons probably all the gem producing global enchantments should be in here.