This text is a comprehensive guide to the Osireion, also known as the Cenotaph of Sethos I. It provides a detailed overview of the discovery and exploration of the underground structure located within the temple complex of Sethos I. The author examines the construction, materials and architectural features of the Osireion, drawing on reports and plans by earlier Egyptologists such as Caulfeild, Murray, Naville and Frankfort. A central point of discussion is the dating of the structure, with the author highlighting evidence for its construction under Seti I and refuting older theories. In addition, hypotheses about the function of the Osireion as an Osirian burial site and the complexity of its water hydrology, as well as potential, as yet unexplored aspects of its layout, are discussed.