Long ago, in the time before the Aether and the Terra were separate realms, humanity was much different than it is now. It's people forgone technology, instead favoring magic over science. The greatest men among them were those who could harness the Aether do to marvelous things. Some could create mountains, some vainly changed their appearance as easy as you would change a pair of clothes. Others went as far as creating new forms of life. However, all feats of magic, however big or small, had one thing in common: The chaos of the Aether had to be combined with the order of the Terra. It was the single unspoken rule of the land, and none dared to break it.
However, like all laws, someone eventually did dare to ignore them.
One summer, a powerful sorcerer of the time, known to all as Galath the Arcane, attempted to create beings of pure Aether. What came to life were creatures of pure, unrestrained power - What came to life were the Sentinels. Each one was reminiscent of a Terra creature, though could harness the pure chaos Aether better than any being.
At first, they watched over the creatures of Terra. To the north, Fengalon the Tempest roosted on the highest mountain, his keen eyes seeing everything in his domain. To the west, Stoldark the Tidal dwelled under the deepest ocean, his slumber only disturbed to protect his territory. To the east, Moa Gault the Inferno patrolled the forests, incinerating anything and anyone he deemed a threat. And to the south, Gurdijeff the Earth-Shatterer burrowed under the hottest desert, guarding his people from the shadows.
After a time, however, Terrakind began to bore the Sentinels. Beings of Chaos can no more dwell in a land of order for long than a fish can in the desert. Not content to dwell in one place and watch, the Sentinels began to make the land more chaotic - Days and Nights began to last for months, water turned to sand, sand to trees, and trees to water. As the four Sentinels rebelled against the Terra, the worlds began to split, Aether and Terra becoming their own realms.
For fear of what he has done, Galath worked night and day to undo his creations. He tried creating more sentinels to keep the originals in line, but that only added to the chaos. His attempts at relocating them only tore the realms further apart. Nothing he tried on his own worked. Until one day, a simple runeist, a peddler of runestones containing nothing more than simple cantrips for the children, showed up at his doorstep, sparking an idea in the sorcerer's mind.
After a month of preparing, and another of forging more potent runestones, he set out to seal the Arcane creatures into prisons of pure terra. One by one they fell, the forces of Terra slowly returning to order as a result. Until at last, the original Sentinels themselves were captured.
The damage to the realm could not be undone, however - The Aether and Terra were forever torn apart, held together solely by the runestones. Galath, dismayed at the destruction he had caused, took his own life after scattering the runes across the world, only to be found by the most adventurous of creatures.
The wold would never be the same, but perhaps that was for a best. As for why, well. That's another story for another time.