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.
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.