Third planet

The Third planet — also called the Planet of Evil by Romana and the Fourth Doctor — was an artificial planet used as a base by the Shadow in his mission to gain the sixth segment of the Key to Time.

The Shadow positioned the Third Planet between Zeos and Atrios so neither could be seen from the other. It was destroyed when the Fourth Doctor rerouted missiles fired by the Marshal of Atrios's Command Capsule into it. (TV: The Armageddon Factor)