History
Zeos was governed by the supercomputer, Mentalis, created by the Time Lord Drax. Mentalis was programmed to self-destruct if threatened; the explosion would destroy Zeos and Atrios. The Doctor and Drax managed to disable Mentalis just before its countdown sequence completed.
The Planet of Evil blocked a view of Atrios from Zeos and vice versa.
The Doctor diverted an attack by the Marshal of Atrios away from the planet into the Planet of Evil. (DW: The Armageddon Factor)
It is unknown what happened to the original inhabitants of Zeos, whether they were wiped out by the Shadow or abanonded the planet.