The Desolation Wars
For ages the balance held. Then it broke.
The wars remembered collectively as the Desolation Wars raged for hundreds of thousands of years. Dral'gath, the Lord of Shadows, opened ways from the Dark Realm and sent his creations upon Aereth. Some were monsters without reason. Others wore forms disturbingly close to those of the people they hunted.
The conflict divided even the Tuatha Dé Danann. Old rivalries became wars, and some fell wholly to the Shadow, turning against the Ddraig, Drakeri, and even their own kin. In later ages these fallen ones would be remembered as Dark Lords - Shadowborn beings whom modern people might call demons. The Shadow found fertile ground wherever distrust took root.
The First Morrighan
Out of that devastation came one of the most enduring alliances in Aereth's history: the Morrighan. Ddraig and Drakeri joined in flight, combining dragonfire and strength with the speed, weapons, and arcane gifts of a mounted warrior.
Together with the remaining powers of the Tuatha Dé Danann and those Sídhe who chose to fight, they drove the Dark Hordes back.
The Sealing of the Abyss
Victory did not mean destruction. Some beings of the Shadow could not be slain by any power then known. The Tuatha Dé Danann therefore built arcane vaults beyond the ordinary world, prisons within the void that became known as the Abyss.
Most were hidden beneath a remote western reach of Gaia. The land above them was torn apart and drowned beneath the sea, and the surviving wards were intended to hold forever.
When their work was done, the surviving Tuatha Dé Danann departed Aereth through their gates, save those who remained as the divine guardians of the Elder Races. Those who had fallen to the Shadow remained as well, though they were no longer counted among their kin. Some of the ancient passages left behind were damaged and unstable.
They would trouble two worlds for ages to come.