The Janus Coin, with the image of the two-headed Roman god on it, was the focal point of the process used by the Regents to separate the consciousness and physical form of H.G. Wells, and store the former in a manner which allows for her mind and personality to be projected and to interact with others.
Anything the person holding the coin remembers is instantly forgotten. It can cause the user to forget certain things or can cause full blown amnesia. However, the memories forgotten by the user are stored on the coin like the files of a computer are stored on disc. The coin can be placed in a black sphere that will project a holographic image of the user that'll remember everything the real user has forgotten. The user can reverse the effects of the artifact by holding the coin again.
It appears in, and is central to the plots of, "Emily Lake" and "Stand", although by inference its effects are seen throughout the third season, as the agents speak to Helena on several occasions whilst she is "detained" in this manner. It might arguably be kept so well hidden because, like bronzing, it is almost the very definition of "cruel and unusual punishment", and could therefore represent a massive threat to the secrecy necessary for Warehouse operations, were its existence to become known.