During the last days of the World-That-Was, Slaanesh glutted himself on countless aelven souls. The last he consumed were those who worshipped Mathlann, God of Storm and Sea, who had hidden their souls in the depths of the world's oceans. Eventually, however, they too were found and devoured.

During the Age of Myth, the aelven gods Tyrion, Teclis and Malerion, aided by Morathi sought out and captured Slaanesh. They then began the arduous task of extracting the half-devoured souls of Aelves from the Dark God's stomach. First to be saved were those aelves who had been last devoured, and Teclis claimed them and from them created the Cythai. Teclisdesired for the Cythai to become the greatest and most noble of their kind, in the image of the noble beings he remembered from the World-That-Was. He built the city of Léiriú in the Realm of Hysh for them, and taught them of the history of their people and the arcane arts. In time, Teclis hoped that the old gods of the aelves would revive, and the Cythai would worship them.

However, he soon realized something was amiss. The Cythai became insular, fractious, and resentful. When Teclis gazed into the souls of his creations, he found dark shadows into which even his light could not see. Teclis cast great spells in an attempt to discern purify the souls of the Cythai and dispel the darkness, but many collapsed into madness and the rest fled for fear that their creator had chosen to kill them all. In desperation, they used the magics taught to them to seek the depths of the oceans - where their souls had last felt safe.^[1][5a]

Beneath the Gealus Ocean in Hysh, the Cythai - now calling themselves Idoneth, from the aelven word for "deep seclusion" - built their first city, Gealrachi. However, the factiousness that had plagued them above the waves still persisted, and when they discovered the underwater realmgates known as whirlways, the factions left Gealrachi and founded their own Enclaves beneath the seas across the Mortal Realms.