In the time even before the Ancient God-Queens, there was a great , which apparently surpassed even them in abilities. Very little is known about it, because it collapsed long before the ascent of the Queens, and even they had to rely on scant archeological evidence. The moniker is just a shorthand due to current lack of a proper name, or any meaningful knowledge, really. Despite this, there are indications that this civilisation is somehow central to the plot - it's theorized they were the ones to create Queen's Mirrors, their major site is on the island which is also the center of new spread of mutagenic "cursed waters" (sounding much like River Dyne) corrupting the secret forest - the dwelling place of Great Cetacean Song Keeper (with whom they also cooperated), as well as the apparent destination of Madwa Korel, working (presumably) on the Other's orders, and Skifander-obsessed Ariadne Steelgarter.
Also, either theirappear to have a difficult relation with linear time, or someone was covering them up (or both).