It serves no other purpose but gives meaning and direction to the plot.

And the DEM comes later to add interest and a twist.

So it would come later.

Be careful with it.
Some say it is lazy writing.

"Biggest McGuffin I ever saw," he agreed.

Then they fired up their Deus ex Machina and resolved the entire plot.

Even an obvious gimmick used well is a worthy plot device, so don’t give my admonition more weight than it merits.

If you are creative, and a good writer, write the story in your head, the way you want to write it.
Don't get bogged down in technique or what other people think you should do.

You are the artist, create your story.