The Founding and Fall of the Kingdom of Numeria
After Atlantis and Avalon had been taken, those kingdoms who had wished to overtake them set up their own city on the perimeters. This city was only a shadow of the previous, but was a sight to behold in its own right. But this city was set up to dominate, to rule, and as it goes with an city with coercion at its center, it eventually fell. It was the great wave that ultimate destroyed them. The remaining sidhe had been also corrupted by this city. Those who were left that still inhabited that land were reborn, as their curse was to be ever born again until they fulfilled their purpose. These naturally lost all recollection of their previous lives, and had to begin again in the way all natural creatures come into this world.
The Numerians were long lived, but once they began having children, they lost the sight of the realm and faded. They built a shadow version of the beautiful city of Avalon, but it was built on a corrupt foundation.
As the spells of protection laid by the various sidhe were initially complacent, the places they built weren't always hidden completely from the world at large. All peoples slipped in an out of these lands for a time, and were difficult to differentiate from the primary world, until they became more defined and separated as they learned from experience not to trust outsiders.
And so it was that it was only after this period of destruction that even the forward looking and thorough cloaking spell of this island came into full effect.