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 using the Doctor's TARDIS. 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)
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 worshipped Horus, aimed to locate the artefact and find the Black Pyramid in order to reawaken their worshipped god. (PROSE: Scarab of Death)
After learning that one Osiran, Anubis, still remained in the universe in the 21st century, the Tenth Doctor took the Eye of Horus from its storage as a precaution. (COMIC: The Infinite Corridor)