The Darzun were an ancient race in the Alpha or Beta Quadrant at some point in the past. They were determined to be very old and advanced, as well as very warlike.
An abandoned Darzun city was discovered by a Federation expedition to Far Pel VII circa 2287. Numerous weapons were found among the ruins, though no trace of the inhabitants' remains.
In 2367, the USS Marco Polo encountered four robot vessels in the Beta Marada system, which were consistent with Darzun technology, and theorized to have been left by that race sometime before they disappeared from the sector. (TNG comic: "Thin Ice")