Revolution
The Orcs rebel, and the other races take their chance to throw off the shackles of the Empire.
The Orcs/Goblins learned enough magic and stole the ritual to create more of themselves. This allowed them to declare their independence and not be mindless soldiers of the Empire. Without their army of super soldiers the other races, and the lower classes rebelled. This resulted in a bloody civil war that lasted decades, but finally the Aeslurin's were beaten back.