The Slaver Empire was a large galactic empire over one billion years before 2269. The Slavers were known as the masters of all intelligent beings in most of the galaxy until one species revolted.
In the war that followed, the Slavers and all of their subjects were exterminated, wiping out most of the intelligent life in the galaxy.
All that was known, and all that was left of those lost civilizations, were their stasis boxes, in which several advanced devices were found. The first one found contained a flying belt, the second a disruptor bomb, and one found in 2269 contained a weapon of a Slaver spy. (TAS: "The Slaver Weapon")