The Black Pyramid was a structure on the planet Phaester Osiris. It was the tomb of the Osiran Horus.
It was in this pyramid that Horus supposedly slept in his sarcophagus after defeating Sutekh. The life support system was faulty and the Osiran lord died ca. 1500 AD. The Cult of the Black Pyramid was dedicated to finding and awakening their lord. They found the Black Pyramid in July 2541 AD when a jeweled scarab was discovered by archaeologists. However, they also learnt that Horus had died long ago. (PROSE: Scarab of Death)