Taking the right-hand passage from the Temple's southern wing leads to the temple's cenotaph (false tomb) that was located some 15 meters lower. The top of this structure is actually below ground level. This is the almost unique structure that today we refer to as the Osireion.
It has been suggested that the architecture of the Osireion was inspired by the Valley Temple and causeway of Khafre at Giza, which by the New Kingdom was buried underground but was still accessible and known to the Egyptians as a tomb or shrine of Osiris. It is carefully aligned with the temple proper, and enclosed within a mud brick wall integrated into the outside storage magazines that are nested within the cavity left by the main temple's L-shaped configuration.
In effect, the main temple acts as a mortuary temple to this false tomb. Hence, no discussion of the temple proper would be complete without covering the Osireion as well.
The structure was built in an excavation in the sandy clay stratum of the desert, with almost vertical sides. Two parallel limestone walls running over the eastern room of the structure served as retaining walls for the sand bed upon which the temple was built. The foundations are cut many feet below the current level of the water table. While this structure was built by Seti I, it was his grandson, Merenptah, who was responsible for its decorative theme. A much simpler structure than the main temple, a long passage, with its limestone pent roof, decorated with scenes and text from the Book of Gates and the Amduat leads through the mud brick enclosure wall at the northwest side of the complex. At a wide point in the passage, it makes a sharp left turn before leading into a long, transverse hall that has a corbel roof cut in the shape of a pent roof beneath. This in turn leads to a monumental hall. While this hall was only partially roofed around its sides with an ambulatory, it is not unlike the massively pillared chambers of the royal tombs in the Valley of the Kings. Measuring 30 x 20 meters, it is made from granite and appears anachronistic, though well suited to its purpose.