The Ancients are one of the four great races of the Myriad Shadowrealms, the Earthlords being their precursors, then the Archons and the Elders coming afterwards, Humani being the race that currently populates the earth. The Ancients' most famous accomplishments were the grand city-colleges they constructed. Though the Ancients mysteriously disappeared during the De Danann Empire's rule of Danu Talis, their knowledge was kept secure within the Great Library of Danu Talis, who's collection of lore was so vast it would take a lifetime to read all of the works of literature on a single shelf.
Thousands of years after the Great Fall, Dr. John Dee convinced his associate Virginia Dare to aid him in his rampage against the Elders. While Dare was promised dominion over all Shadowrealms, Dee in turn would be granted the Great Library, which held the knowledge of all the Great Races, including the Ancients. It has been hinted by Gilgamesh the King that the Ancients were in fact immortals from an incredibly distant era.
The ancients are described as being similar in shape and size to the Elders.