Farvarshi (Mazd.). The same as Ferouer, or the opposite (as contrasted) double. The spiritual counterpart of the still more spiritual original. Thus, Ahriman is the Ferouer or the Farvarshi of Ormuzd— “demon est deus inversus”—Satan of God. Michael the Archangel, “he like god”, is a Ferouer of that god. A Farvarshi is the shadowy or dark side of a Deity—or its darker lining.
Source: H.P.Blavatsky - The Theosophical Glossary