Actually the Hidden are also referenced quite a bit in game during the Taken King campaign. If you look at the transcripts [url=http://www.ishtar-collective.net/search/the%20hidden]on this page[/url] you can actually see the videos in question, but yes WeakenedHook is correct, they are a spy network that reports to Ikora Rey which the Queen is aware of and possibly connected to in some way and Eris Morn is a member, as was Eriana-3.