Atrios

Atrios was a planet devastated by nuclear war with its neighbour Zeos and ruled by a monarchy represented by Princess Astra. The war effort was led by the Marshal of Atrios.

The third planet blocked a view of Zeos from Atrios and vice versa. The Doctor destroyed the Planet of Evil by rerouting missiles fired by the marshal originally directed at Zeos. (TV: The Armageddon Factor)

Without the energy of the segment to keep her together, Astra's role as the sixth segment was passed on to Romana after the latter's regeneration. Astra began to unintentionally drain the life-force from her subject Atrians, causing the extinction of the Atrians. (AUDIO: The Chaos Pool)

In 2050, K9 Mark 2 absorbed the Black Hunger swarm to prevent it from destroying Earth. He planned to evacuate the swarm the next time his was on Atrios because as he explained, the planet had no carbon-based lifeforms. (TV: Black Hunger)