If you played AD&D in the late 80s and through much of the 90s, you probably started at the above image at the start and end of every session… It was the cover of the PHB, after all, by Jeff Easley.
But, let’s try to look past the memories and the association, and focus on the image.
Who is the central figure? Who is the one trailing? Are they allies, or opponents? Consider who is the POV [the one being threatened by the sword]… what is that relationship to the other two? [Why the threatening? Why the horse chase? Who is trying to get away – the POV, or the man/men on horseback?]
While the above image is (obviously) easily used for a fantasy, consider how it could be made to fit into a SF mold.
What happened to set up this moment, and what happens as the rider(s) thunder past?