Corah was a purple planet.
The Tenth Doctor visited this planet and was captured by a military research team led by Professor Slade. By a nearby lake were crude, human-shaped statues which gave off a faint electrical signal. The research team believed that the Doctor had killed a fellow crew member, but when the Doctor asked them to count the statues, there was an extra one. The Doctor believed something in the pool was transforming people. A tentacle attached to a very large green creature emerged and rapidly drained all the moisture from Grent and another crew member, leaving behind a husky shell.
The Doctor offered an endless supply of water in his ship, but the creature pursued them across the rocky terrain. The Doctor and Slade were aided in their escape by McEvoy, who brought them skimmer craft (Sky-bikes). With the creature still in pursuit, the Doctor led them to a faint energy pulse he detected. It came from what appeared to be an underground city abandoned by Corah's once mighty civilisation. The City turned out to be a weather control station, built to stop a great drought, but it didn't work. The creature pursuing them had adapted over thousands of years to absorb moisture and water.
The Doctor re-aligned the machine's vapour density regulator. With a few other tweaks, he engaged maximum particle excitation and... it rained. The creature returned to the pool and the Doctor told Grant and McEvoy that their crew should be re-hydrated and returned to their former selves. The Doctor left an umbrella with them for their walk back. (COMIC: The Guardians of Terror/The Rebirth of Corah)