Currently - this Great Prophet Aes Sidhe is listed as Svartalfar. My first thought was that linked her with Esus, but that doesn't tie in with the Council of Esus which as per the pedia has no priests, high priests or temples (and from that Great Prophet). The other alternative would be a Prophet of Sucellus, perhaps as a member of the Unseelie court - but it doesn't seem right that you would have a Svart great Prophet to someone that they later turned from.
So if we go back to her being a Great Prophet of Esus - could it be that she did much to further the cause of the Council, but in doing so perhaps is punished by Esus?!
Alternatively, would it be stretching things to consider her a Great Thief - the legends of which become revered by the Svart?
Another alternative - and I confess I don't know enough background lore about the Order and Empyrean - is that perhaps at one time they were as one, but through her machinations she helped create a schism to split them, and split their power (which again would tie her back into Esus...)