Your sovereign (maybe some of the champions you teach magic too) can change the wasted land or the land of the opposite alignment into land of your alignment. This costs essence. You can only build cities on land of your alignment, yet if the land already is your alignment you have no need for the Revive Land spell. The kingdoms have the green grass and the fall the black brimstone. Everything else (swamps, deserts, barrens) are all wasted lands.

I was under the impression that they changed this recently? Something about cities being able to be built on non-revived land or some such.