The Cataclysm of Ladon

Following the Colossus War and the near-total destruction of the Titans, the dragons were left as both guardians and custodians of Aiaos. The Elder Wyrms, immortal scions of the Primordial Dragons, had been destroyed by the Colossi, so that their mortal descendents needed to take up their mantle.
The Primordial Dragons decreed that they would look to the centre to hold Aiaos together. They had long since established the dragonborn nation of Ladon at the geographical heart of Aiaos, and intended to make it the anchor by which the World could be maintained. At this time, Aiaos still had only its original supercontinent of Aiaor.
The Colossus War had put a great strain on the world, but even as other civilisations arose across the continent, the Primordials intended that Ladon should be a perfect society, and thus act as a mystical anchor to hold the rest together. In place of the departed Old Gods, the Primordials Hroun, Tropos, Ramad and Lacaga became the gods of North, South, West and East respectively. Arabast and Malcidis took station in the Primal Wild and the Grim Plain, and in the centre, over Ladon itself, Tempus held the balance.
But the Primordial Dragons were made to defend, not to maintain. They chafed in this new role; Tropos and Lacaga became willful and reckless. Hroun, greatest seer of Aiaos, was deceived by eir own visions. Foreseeing a danger of rebellion, e led Arabast in a precipitate attack which brought about the very conflict e had feared. As dragon fought dragon above Ladon, the balance shattered, and with it the stones of Aiaor. Much of the central plain sank below the waves of a new, central ocean. The northern and western coasts were thrust back to the Elemental Boundary, the north folding and rising into the Auroral Highlands while the west tore through the Great Archipelago. Tempus allowed his essence to scatter and reform into the Web of Fate to prevent total destruction, and the surviving Primordials - Arbast and Malcidis - took stations in the Firmament, circling the World and maintaining the Web.
The mortal civilisations were set back centuries by the destruction. All that remained of Ladon were the scattered islands of the Ladonian Chain which had once been the highlands of Auris, and even as the dragonborn clades began to feud over that meager territory, the mortal dragons withdrew from the world. Worse yet, the Eladrin civilisation, ancient and advanced, was housed almost entirely in the Great Archipelago, and was utterly lost.