Helena Petrovna Blavatsky[a] (née Hahn von Rottenstern; 12 August [O. S. 31 July] 1831 - 8 May 1891), often known as Madame Blavatsky, was a Russian mystic and author who co-founded the Theosophical Society in 1875. She gained an international following as the leading theoretician of Theosophy.