Rills were a peaceful and advanced telepathic race.
Biology
Rills were large, scaly creatures. They had powerful skulls with wide, tusked mouths and large eyes. They had several pairs of tentacles, including six with human-like hands. They communicated telepathically and were unable to speak vocally.
Rills had long lifespans, with a five hundred year old Rill being comparable to a ten year old human. They also had a slowed perception of time; weeks seemed like years to them. Normally, they moved fifty times slower than a human and thought only twice as fast as they moved. In order to deal with other species, the Rill could increase their reaction time. Though they could speed up their minds to the rates of humans, they found it uncomfortable. (TV: Galaxy 4, PROSE: Galaxy Four)
Culture
Rills were pacifists, only fighting when attacked and left with no other options. (PROSE: Galaxy Four) They had a political system in which the uglier one was, the more power one attained. (PROSE: Lucifer Rising) In general, though, leadership was unneeded, as the Rill had so little activity and preferred to simply think, leaving robots to do any activities. (PROSE: Galaxy Four)
Female Rills found a large skull sexually attractive. (PROSE: Galaxy Four)
Interior of a Rill spacecraft. (TV: Galaxy Four)
Technology
Rill ships were massive black spheres with vents for propulsion. They were potentially capable of intergalactic travel and were capable of incredible speed. Though the Rills were pacifists, their ships had advanced weaponry and force-shields for protection. They also had devices to convert an atmosphere into one rich with ammonia. Their ships used nuclear power, which they called "Solar".
A Rill within their ship. (TV: Galaxy Four)
Much of the work was done by the Chumblies. These robots did menial labour, defended the Rill and communicated with outsiders. (TV: Galaxy 4, PROSE: Galaxy Four)
History
Early in their history, the Rills vastly increased their lifespans. They came to regret this, finding their lives dull and boring. They slowed down their perspective of time, allowing them to appreciate life more. They also started travelling through space, taking the time to learn and encounter new civilisations.
At some time a Rill ship encountered a Drahvin ship. The Drahvins attacked and though the Rills were more advanced, both ships crash-landed on an uninhabited, doomed planet. The Rills tried to help the Drahvins, even to heal a wounded soldier, but the Drahvins refused their help and tried to take their ship for themselves. When the First Doctor, Steven and Vicki came to the planet, the Rills tried to contact them via the Chumblies. The Chumblies were modified to communicate with them and they came to an agreement. The Doctor recharged their ship with the TARDIS' power, allowing them to escape the planet. (TV: Galaxy 4, PROSE: Galaxy Four)
Rills visited Kirith and were taken to Kandasi, where they were presumably killed. (PROSE: Timewyrm: Apocalypse)