Xenon was a planet used as a base, first by Dorian and then by Avon and his followers. It was, not entirely surprisingly, the location of Xenon Base.
Xenon was an Earth-like planet with a breathable atmosphere. It had once supported an advanced and long-lasting civilisation, but this had been destroyed in a single day's war hundreds of years earlier. Remnants of the old civilisation could still be occasionally found on the surface and beneath it. The older civilisation was said to have lasted 10,000 years and thus it was almost certainly not human in origin.
The survivors of the war regressed to become savage tribes known as Hommiks, male-dominated and prone to hunting women for reproductive purposes. Also on the planet, having come there from elsewhere, was a technologically-advanced all-female group called the Seska.
Dorian chose Xenon as his base of operations following his discovery of a cavern with strange properties. He constructed Xenon Base there and worked with the Seska on developing a teleport system.
Shortly after the arrival of Avon and his crew the last few Seska were wiped out. The Hommik tribe that had lived near Xenon Base moved out of the area immediately afterwards.
Avon and the others fought and destroyed Muller's android on the surface of Xenon.