The Pharos was a pre-Imperial biotechnological device created by an unknown xenos civilisation that harnessed the psychic energy of empathy to aid navigation through the Warp, much like the later Astronomican, though on a far smaller scale. The Pharos was far less powerful of a psychic broadcaster than the former device, and if the Astronomican could be likened to a lighthouse in the Warp guiding the Navigators of voidships to their destination, the Pharos was more like a lantern. However, the Pharos allowed its user to find a path through the Warp, or in some instances, even teleport through the Warp across many light years of space to a location that he or she consciously or unconsciously desired.
A Pharos device was first discovered by the Loyalist Iron Warriors Warsmith Barabas Dantioch on the world of Sotha within the geographical landmark of Mount Pharos in the Ultima Segmentum during the Horus Heresy. It was used by Primarch Roboute Guilliman of the Ultramarines Legion as a psychic beacon to attract Loyalist forces of the region that had been scattered by the Ruinstorm to the Ultramarines' homeworld of Macragge. Cut off from all travel to or communication with the Throneworld of Terra by that great Warp Storm conjured by the Word Bearers Traitor Legion during the Battle of Calth, Guilliman had created his own rump interstellar empire, the Imperium Secundus, to rally the Loyalist forces against the Traitors and maintain the ideals of the Emperor of Mankind. The Pharos allowed Guilliman to gather all of the Loyalist forces to support his new Imperium, including the Dark Angels and Blood Angels Legions.
The Pharos became a source of contention between the Loyalists and Traitors when members of a Night Lords Traitor Legion splinter fleet under the command of Captain Krukesh the Pale sought to claim it from the forces of the Imperium Secundus at the Battle of Sotha. The Night Lords Legion had been scattered since the outcome of the Thramas Crusade and its Primarch Konrad Curze's defeat by the Dark Angels. After learning of the existence of the device, Krukesh hoped to seize it and use its abilities to discover the location of the Nightfall, the Gloriana-class Battleship that served as Konrad Curze's flagship, and reunite the Night Lords Legion so they could join up with Horus' forces for the coming assault on Terra. During the Battle of Sotha, Barabas Dantioch sacrificed his own life to overload the Pharos, disabling the Night Lords fleet above Sotha and allowing the Loyalist forces to prevail.
Unknown to all involved, the sudden burst of psychic power from the device during its overload reverberated across space-time into the vast intergalactic wilderness, where it was detected by the advance scouts of the Tyranid Hive Fleets. Made aware of the existence of sentient life in the nearby Milky Way Galaxy, the Tyranid Hive Mind set the Great Devourer's course towards this vast new source of biomatter. It would take nearly ten thousand standard years for the Hive Fleets to arrive in the late 41st Millennium, but these events of the Horus Heresy had unwittingly set into motion the greatest alien threat Mankind would ever know.
The Pharos was the name given to the Great Lighthouse of Alexandria, Egypt. It was built by the Ptolemaic Egyptian Kingdom between 280 and 247 BC and was between 120 and 137 metres (394 and 449 feet) tall. One of the Seven Wonders of the Ancient World, it was one of the tallest man-made structures on Earth for many centuries. Badly damaged by three earthquakes between AD 956 and 1323, it then became an abandoned ruin. It was the third longest surviving ancient wonder (after the Mausoleum at Halicarnassus and the extant Great Pyramid of Giza) until 1480, when the last of its remnant stones were used to build the Citadel of Qaitbay on the same site. In 1994, French archaeologists discovered some remains of the lighthouse on the floor of Alexandria's Eastern Harbour.