Otto Henne am Rhyn (1828-1914) was a Swiss journalist, historian and Freemason whose comprehensive universal cultural history was a major contribution to the development of the German Kulturgeschichte (History of Civilization) school. His publications were a forceful challenge to the traditional emphasis on political and military themes in German historiography. His greatest work is considered to be the Allgemeine Kulturgeschichte, 8 vol. (1877-1908), an “Universal History of Civilization”, from earliest times to the closing years of the 19th century. Mysteria, subtitled History of the Secret Doctrines and Mystic Rites of Ancient Religions and Medieval and Modern Secret Orders, published in 1895, is considered one of the best-documented essays on the history of initiatory thought and secret societies. This essay, encyclopedic but easy to read, still constitutes a milestone for those who intend to approach the great mysteries of esotericism and initiatory tradition throughout the centuries.