Zeos

Zeos was a planet at war with its neighbour Atrios.

History
Zeos was governed by the supercomputer, Mentalis, created by Drax, a Time Lord. Mentalis was programmed to self-destruct if threatened; the explosion would destroy Zeos and Atrios. The Fourth Doctor and Drax managed to disable Mentalis just before its countdown sequence completed.

The Third Planet 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 abandoned the planet.