Chaos was a planet where the rain never stopped. This fact showed the Fifth Doctor that there was another force was present when he met Amy for the first time, as this force, which was actually the Grace, had stopped time, making it seem that the rain had ceased. (AUDIO: The Judgement of Isskar)
It was located in a hyperspatial fold, making the extreme ends of the universe co-adjacent to each other, making the Atrions and the Teuthoidians fight each other in their respective searches for the Pool. This fold was ironed out when the segments in Zara's satchel came into contact with the warp manifold and other equipment within Professor Lydel's laboratory, even though the Doctor warned him not to put the bag near it.
Abby and Zara later visited Chaos in order to find the Grace so that they could help Kreekpolt save his daughter, Persephone. (AUDIO: The End) Persephone later bought the planet and found a way to stop the sisters power in order to punish them for their deal with her father. (AUDIO: The Dark)