The Shining Trapezohedron serves as a window on all of space and time. By gazing into it, its wielder would summon the Haunter of the Dark, an avatar of Nyarlathotep, as well as (possibly) other Outer Gods. In exchange for horrible sacrifices, the Haunter of the Dark would reveal other worlds and arcane knowledge.
The Shining Trapezohedron manifests as a crazy angled, mirrored stone. It is said to have been created on the plane of Yuggoth and brought to Earth by the Yuan-Ti before the emergence of the human race. When the serpent-men's empire collapsed during the fall of Lost Farbosheia, it was eventually discovered by the first humans of Great Mbo, specifically Nephren-Ka.
The Shining Trapezohedron was researched during Nephren-Ka's reign by the corrupted priest of Thoth, Ra-Blajkess, who deciphered a number of ancient texts concerning the stone. He attempted to summon Yog-Sothoth with the stone, and saw him re-creating universes, but the attempt drove him mad. His crazed rantings were preserved as part of a cursed tomb of dark magic that eventually found it's way into the possession of Carnamagos, who mentions it in his Testaments of Carnamagos. Carnamagos at one time considered questing for the stone, but considered it too dangerous. He considered it an expression of Mirror Magic, which explains it's affinity with prophecy and it's tendency to drive the wielder mad.
According to one account, the Shining Trapezohedron can be used as a focal point to open gates between alternate worlds and dimensions. Its light can heal, destroy, and transform, allowing entities such as Nyarlathotep to shift between human and non-human form.