[Makinus]

About the fact that some nations lose magical power along the eras while others remain strong or even get stronger:

I think that the magic weakening in as the eras advance is not the only force at work, but there are 2 other forces in action: the increasing of human population and the flourishing of technology.

The magic weakening is not too pronounced, but humans start to displace old, more magical races and technology advances making several uses of magic obsolete.

Some nations get weaker magically because either their more magical pop is displaced by humans and humans are not naturally magical

or they start using more tech and less magic.

Some other nations (Utgard is a good example), get more humans in their pop but these humans adopt a lot of the magic culture, without any weakening of the magic and even strenghtening it.

Other nations get magically boosted because of events that change the nature of the nation (La Ryleh) or absorb two magical cultures that were two different nations in previous eras...

Maybe i'm imagining it but there was not a canon document somewhere where we had the general dom3 story as the eras advance?