In the book Dune Messiah, I read:
"The Emperor has said I must die if I set foot on Dune," she said, making a last desperate effort. "You spoke of this yourself. You are condemning me if you take me down there."
"Say no more," the Qizara ordered. "The thing is ordained."
A common alternative is "This is an order", but here it uses the word ordain. I looked it up in the dictionary and found that they have similar meanings:
Definition of ORDAIN
1 : to invest officially (as by the laying on of hands) with ministerial or priestly authority
2a : to establish or order by appointment, decree, or law : enact— we the people … do ordain and establish this Constitution — United States Constitution
2b : destine, foreordain
intransitive verb : to issue an order
So, when should I use ordain instead of order?