Is this sentence essential or nonessential? this theory, which may at first seem like a old wives tale, actually has some basis in fact.

The sentence itself really doesn't give us a hint. It can be a very good lead in statement for something that discusses cures currently classified as Old Wive's Tales. If you google the statement it comes back with all kinds of entries some of them actually relevant.

I would say that it can be essential. It depends on the context.

the term that describes the best dear evan hansen

it ends with a marriage, but also with prospero giving up his power.

the tempest is a play written by william shakespeare between 1610-1611, it narrates the story of prospero a magician that sets a storm to wreck a ship, this sets the story to kill the king of naples. while it is a romantic style play, it isn´t traditional because of the ending, it end with the marriage and the magician giving up his powers.