Dark Sun: The Scorched World of Athas
The Green Age began when the halfling nature masters used the Dark Lens and the Pristine Tower to drain energy from the sun, turning it from blue to yellow, in order to stop the brown tide destroying the seas.
The Green Age contains several different periods.
Starting with the use of the Dark Lens, around 14,000 years ago, until the discovery of magic, about 8,000 years ago, the rebirth was a time of growth and prosperity. Under the now yellow sun, new races filled the mountains, forests and fertile valleys, unscathed by the receding waves. Great cities spring up and religions develop worshiping the gods. Psionics develop during this age and nearly all life has some psioinic talent. An unprecedented age of cooperation and standard of living marks this age.
The Time of Magic
About 8,000 years ago, a misshapen pyreen, Rajaat, discovers the principles of magic on Athas and publicly teaches the way of preserving, while secretly researching and teaching the ways of defiling magic to a select group of humans.
Around 3,500 years ago, the most powerful of his followers gathered at the Pristine Tower and used the Dark Lens to imbue them with imortality and great power. This process turned the sun from yellow to red and began the Red Age.