The Stariionae's Enemy is an unidentified nameless race of machines (an artificial intelligence) that make extensive use of drones, controlled by "Overlords". They are quite malevolent and ruthless in their pursuit of Stariionae, in their effort to exterminate the spacefaring race.
Like the Stariionae, they are nomadic and entirely space-based, with no need for planetary support bases.
Beyond that, virtually nothing is known with any certainty.
It is possible both the Stariionae and this ancient Enemy of theirs were created by the same precursor race eons ago, as a creation and some reactionary control measure... or perhaps they were created by two belligerent races as war machines.
But these are just guesses. The Stariionae themselves seem to have no knowledge of their origins, the origins of their Enemy, or when or why the two began fighting.