What would a term for diminishing of magic, diversity, beauty, and power be?

I am trying to figure out what the proper literary term would be for the following description.

In many books and films there is a theme where the world is or was full of magic and beauty and then that age ends and the following is more mundane.

Sometimes this is accompanied due to results of an evil event and other times just from the slow approaching of time.

An evil occurrence example could be something like in Star Wars: A New Hope, when Luke is in Kenobi's little house,

An elegant weapon for a more civilized time. For over a thousand generations the Jedi Knights were the guardians of peace and justice in the Old Republic. Before the dark times, before the Empire.

An example of time could be in The Lord of the Rings at the end of the 3rd age (after the war of the ring). Most of the elves are either leaving or have left, Gandalf leaves, later into the age the Ents become..treeish, and the dwarves vanish from mans knowledge. All that is left is men.

So while Men flourish all the other races diminish. Theres even a scene in the movies where Frodo and Sam spy some wood elves leaving middle earth and Sam says,

I don't know why, but it makes me sad"

So my question is, what would a term for this..diminishing of magic, diversity, beauty, power be?

It's kind of like Nostalgia in a way, not sure what the best term would be..

The term my father would use (to describe the diminution of his own powers from previous heights) is decline.