The Coming of the Chaos Emeralds was an event in which occurred nearly 12,000 years ago during prehistoric Mobian history, which resulted in the extinction of the Mobosaurs and the birth of the green Chaos Emeralds on Mobius.
The Coming of the Chaos Emeralds was a result of an event which took place some 2,000 years earlier. The Xorda, a warmongering alien race, attacked the planet Earth using their Gene Bombs to cause the devolution and destruction of the human race. Several decades after the attack however, the bombs reacted with the planet, resulting in billions of tons of dirt and ash to be hurled into the atmosphere. For over a thousand years, the planet remained scarred and untouched by sunlight, until finally the sun's rays penetrated through the layers of ash and dirt.
Following this, the Mobosaurs emerged as the new dominant species on Mobius.
Coming of the Chaos Emeralds
The residual energies of the Gene Bombs gradually met and reacted with the beryl deposits deep within the planet's surface over thousands of years. The result was the creations of the Chaos Emeralds, and their massive eruption from beneath the planet's crust.
As the eruptions occurred and the Chaos Emeralds descended back down to the surface, the planet suffered massive geological changes, resulting in the extinction of the Mobosaurs. There were however 3 Mobosaurs, later known as the Ancient Walkers, which managed to survive, having obtained Chaos Emeralds and gained a vast amount of powers, wisdom and insight from their magical powers. A fourth being known as Mammoth Mogul happened to have one of the descending Chaos Emeralds embed itself in his chest, granting him immortality.
With Mogul now immortal, and possessing a far greater intellect than his Mobian counterparts, he went on to create the Order of Ixis, a group of beings who mastered the Ixis Magicks and partially the Chaos Force and were loyal only to him. During their bid for world domination, the rapidly evolving Echidnas grew to act as a rival power. The clash of the two resulted in the Forgotten War, between the Order of Ixis and the Albion Knights of Aurora.
Since then, the Chaos Emeralds have been a source of power and a reason for conflict throughout Mobius. From the ascension of Angel Island, to the reign of Dr. Robotnik and his bid for world domination, to Mammoth Mogul's attempt at bringing about his world order. Recently, the Great Harmony caused by the Chosen One (revealed to be Miles "Tails" Prower) resulted in all of the Chaos Emeralds in the universe to be sent into the Zone of Silence.