The Eldar are an ancient race of natural space travellers. They are wanderers among worlds, and they travel from world to world, observing. They do not act or change the universe; they are observers only. 

The Eldar have travelled extensively and seen the beginnings of life, the emergence of intelligence, the death of stars, and the many wonders of countless worlds. Life and intelligence were of particular interest to them; they sought to understand life in all its forms, so that they would understand themselves, their origins, and their purpose. They believed that when they had a complete understanding of existence, they would understand how to pass beyond and enter a new state of existence in which they were free to act as well as observe.  

The Eldar discovered Arcadia long ago, when it was in its infancy. Life there was flourishing, but no intelligence had yet emerged, and yet the Eldar had never before encountered this world. This was unusual. How had such a place remained hidden from them? The Eldar were aware of the galaxy, and especially aware of life bearing worlds. They learned that there was a natural force, unknown to them, which inhibited their perceptions and had hidden this world from them. They knew that they must understand this force to understand the universe, and sent hundreds of their people to the Arcadia.

Arcadia was a wonder of wonders to them, for they found that by turning their minds to the task, they could go beyond mere observation -- they could act, they could create. In their joy, they created works of power and subtlety, calling forth life forms in imitation of those they had seen on other worlds and erecting mighty edifices to their own power. They brought forth the Elves and taught them how to use Arcadia's Magic, the Dwarves and taught them the Craft of 1000 worlds, and the Saurians, and taught them obedience.

When their joy subsided, they came to a realization: they were cut off from the other Eldar. They could not contact the minds of the others. They could not leave Arcadia. They were trapped; bound to Arcadia by the very acts of creation in which they had indulged themselves.

Over time, their isolation drove them mad. Now aloof from their creation but still bound to it, they became worshipped as gods by Dwarves, and Saurians. Humans too, who came last, came to worship them. Only the Elves recalled the true nature of the Eldar.