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 flashing red gem and was located in a pyramid on Mars which was safeguarded with multiple traps, including Osiran service robots loyal to Horus.
In 1911, Sutekh attempted to destroy the pyramid on Mars with a rocket. When the rocket was stopped by Sarah Jane Smith, Sutekh sent Marcus Scarman and an Osiran service robot to Mars. There Marcus destroyed the Eye of Horus which freed Sutekh from the pyramid on Earth. He was later defeated by the Fourth Doctor. (TV: Pyramids of Mars)
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)