The Intaro is a figure of myths, sagas and legends. Some scholars say that she has always been a part of Athyria, a gift from its creators to the children of the Eternals.
Others say that the First-created is an echo of the melancholy that was felt when the creators of Athyria surrendered the world to their children and turned away to face their inevitable death.
Then as now, Rhiannon embodies the noblest virtues that mortals might attain: heroes laid their swords at her feet and poets created works in her name. She was the pinnacle of numerous quests and the source of all honor and wisdom and remains as such in the minds of the tribes of Athyria even after her death.
More on this in Athyria Wiki: Intaro Rhiannon