They once ruled the Skonnan Empire, controlling hundreds of planets, but there was a massive civil war and the empire collapsed. This caused the death of nearly all of their scientists and men of learning. The military gained control of Skonnos, and their technology decayed, leaving them with a broken down fleet of starships.
The Skonnans were discovered by the Nimon, who promised them a new empire in exchange for sacrifices of living beings and hymetusite. They gathered these sacrifices from Aneth, an old subject of the Skonnan Empire. Though Skonnos and its new leader Soldeed obeyed the Nimon's will, the Nimon were eventually defeated by the Fourth Doctor and their black hole generator was destroyed. The Skonnans' hopes for a quick imperial revival and a return to military glory ended for the time being.
The Skonnans were a warrior race obsessed with glory and military conquest to the point of religious fervor. Many people wore elaborate martial costumes consisting of black uniforms with black helmets sometimes with tall feathered plumes. (TV: The Horns of Nimon)