The Green Age of Athas began after the waters of the Blue Age had receded, leaving fertile valleys and plains that quickly became verdant. The rhulisti began to use their knowledge of life-shaping to transform themselves into other races - such as humans, giants, dwarves, pyreen and tari - as they spread out into this new world.
One particular pyreen, known as Rajaat discovered the secrets of magic, powered by the life energy around him. He taught preserving magic openly, and defiling magic (which is more powerful, but at the cost of nearby plants) in secret to a few favourites. Of these favourites, he selected the most powerful in both magic and psionics to be his champions. The champions were all human, as they seemed best able to adapt to the magics of the Pristine Tower that he would use on them to shape them into living weapons.
Each champion was assigned a new race to be wiped out, so that the world could be returned to the Blue Age. And thus began the Cleansing Wars, in which the champions' magic turned the planet into a vast desert.
Eventually, the champions discovered that Rajaat intended to destroy humans as well - including them - and leave the world for the rhulisti (now known as halflings) alone. They turned on him and imprisoned him in The Hollow. The Champions used the Pristine Tower again to become sorcerer-kings, and they each took or built a city to rule. As they did this, the sun turned from gold to blood red, ushering in the modern, or brown, age.