Dark Sun: The Scorched World of Athas
The Blue Age is the oldest known time period of Athas. During this age, the current calendar was created, over 14000 years ago.
The Athasian sky was an azure hue, under a sapphire sun. The majority of the world was covered in water and halflings rose to build an empire led by nature masters. These halfing nature masters were powerful artificers and elemental clerics. A blight, a great brown tide developed and the seas began to die, marking the beginning of end of this age.
The halflings gathered at the Pristine Tower and, using the Dark Lens, performed great magic to stop the brown tide. The magic drained energy from the sun and turned it from blue to yellow. The magic release stopped the brown tide, but also created many other changes to the world, starting the Green Age.