The Eldar were the Elves who were closest to their masters the Valheru. They were their Lorekeepers and Librarians and travelled with them to other worlds on the backs of the Great Dragons when they raided across the Universe.
Before the end of the Chaos Wars one group of Eldar fled Midkemia to the world of Andcardia and would eventually become the Taredhel.
When the Chaos Wars ended another group of the Eldar went into self-imposed exile underneath the polar ice of Kelewan and establised an outpost called Elvardein to guard against the day that the Valheru returned.
Pug found mention of them in the library of the Assembly merely as Those who Watch. Following this discovery he traveled to the polar region mentioned and discovered their Elvardein. It was there he was taught the art of spellweaving by the Eldar, whose leader was Acaila.
When the Valheru were defeated at Sethanon the Eldar returned to Elvandar from their self-imposed exile.