The Eye of Horus was an Osiran device that beamed a signal that suppressed the renegade Osiran known as Sutekh and held him prisoner inside a pyramid on Earth. This artefact resembled a round gem and was located in a pyramid on Mars which was safeguarded with multiple traps, including Osiran service robots loyal to Horus. (TV: Pyramids of Mars)
It was said that Horus and the Osirans that supported him left the eye just out of reach in order to make Sutekh's punishment more tortuous. (PROSE: The Sands of Time)
The Cult of the Black Pyramid, a religious organisation that worshiped Horus, aimed to locate the artefact and find the Black Pyramid in order to reawaken their worshiped god. (PROSE: The Curse of the Scarab)