This is a substitute term describing a civilization that apparently is not native to the Milky Way Galaxy and has periodically visited the Milky Way Galaxy in prehistoric times. The earliest records of the Dark Ones come from the First League . (The first known Multicultural association in the Milky Way Galaxy over 4 Billion years ago)
Due to the extreme time intervals, there is little more than legends and myths of this time.
(That changed as surviving First League civilization was discovered by the crew of the USS Tigershark: See: The Clonck)
Other incidents of the Dark Ones visiting have been observed and recorded by the Pree and the UNI. It is believed that the sudden demise of the Pree and the apparent flight of the UNI, abandoning the Milky Way Galaxy are closely tied to that event.
The Narth do have some collective knowledge about the Dark Ones, but generally do not want to share this information.
The Seenian (Celtest) a technological highly advanced civilization, ruling over much of the Milky Way galaxy about one million years clashed with the returning Dark Ones in an all-out galactic wide war. They enlisted the Saresii, the Nul, and several advanced civilizations as allies. The Seenian civilization was completely destroyed and only the Saresii and the Nul survived. However, the Dark Ones punished the Nul by destroying their sprawling empire and throwing the Nul back into the Stone Age. The Saresii also lost over 25,000 colonies that were completely destroyed. Only Saresii Prime survived severely damaged and gave up on space exploration for nearly 800,000 years isolating themselves.
While there are ruins and artifacts associated with the Seenians and other civilizations that fought the Dark Ones, there are no known artifacts or even images of the Dark Ones.
The legend claims that the Dark Ones were a Civilization of Tech Level 12 (the highest possible technological level any civilization could attain) and ruled over thousands of galaxies, and over several parallel universes at the same time. It is said the Dark Ones were led by an entity known as Invictus Rex