Template:Inmythos The Hidden Ones are beings who worship the god Zulchequon. The veiled leader of the Hidden Ones is Tsunth. Possibly they are the Nyghan Grii or the Moon Beasts.
The members of the Order of Assassins were known as Hidden Ones in Pre-Cataclysmic times.
Source
This is from "The Three Bladed Doom" a Conan story adapted by L. Sprague de Camp from a Robert E. Howard story. As Howard places the Hyborian Age in the Mythos frame, the reference is included here.