The Pharos was a pre-Imperial biotechnological device created by an unknown xenos civilisation later identified as the Necrons 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 beacon 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.
The Pharos was one of many such artefacts scattered across the galaxy. It was built of blackstone, and the array of Pharos devices was originally intended by the Necrons to provide a way for them to achieve rapid interstellar communications and FTL travel.
Like most other Necron technology based on blackstone, the Pharos was able to negate the effects and presence of the Warp, or to enhance it.
The Pharos array was composed of eight nodes located across the galaxy, each of which was powered by one of eight C'tan Shards. The Pharos located on Sotha was powered by a shard of the C'tan known as Zarhulash the Potentate.
When the Necrons entered their hiberation millioms of Terran years ago, the Pharos array went off-line. By the time the Age of the Imperium dawned with the Great Crusade, the only known surviving Pharos was located on the world of Sotha in the Ultima Segmentum.
Belisarius Cawl later stated that he had discovered another Pharos elsewhere, but it had been destroyed long before.
A Pharos device was first encountered by humanity when the Loyalist Iron Warriors Warsmith Barabas Dantioch arrived on the world of Sotha during the Horus Heresy and found it had been constructed within the geographical landmark of Mount Pharos, which gave the device its name.
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 the great Warp Storm called the Ruinstorm that had been 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 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 using its energies and allowing the Loyalist forces to prevail.
After the end of the Horus Heresy, Roboute Guilliman ordered the Pharos destroyed. Thousands of tonnes of xenos machinery were removed from the mountain and its tunnels were filled with millions of tonnes of ferrocrete.
The Primarch appointed an Ultramarine officer named Oberdeii as the Warden of the Pharos and Captain of the Ultramarines' 119th "Aegida" Company, who were sworn to protect Sotha from whatever secrets had been buried in the Pharos.
Beacon for the Hive Fleets
Unknown to all involved, the sudden burst of psychic power from the device during its overload at the Battle of Sotha reverberated across space-time into the vast intergalactic wilderness, where it was detected by the advance scouts of the Tyranid Hive Fleets.
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.
Over ten thousand Terran years later, the birth of the Great Rift that bisected the galaxy at the start of the Era Indomitus reactivated the Pharos. This was discovered by the Scythes of the Emperor, who still guarded the world after its destruction by a Genestealer Cult infestation and Hive Fleet Kraken during the Second Tyrannic War.
Archmagos Dominus Belisarius Cawl led an expedition to Sotha with the aid of the Ultramarines and the remaining Firstborn Space Marines of the Scythes of the Emperor to study and perhaps use the Pharos' technology to close the Great Rift and defeat Chaos.
The expedition came under assault by Genestealer Cultists who had infiltrated the Chapter Serfs of the Scythes of the Emperor. Ultimately all of that Chapter's remaining Firstborn Marines were slain alongside their Chapter Master Thracius in combat with the Genestealer Patriarch whose cult had first called Hive Fleet Kraken to destroy Sotha.
After reaching the Pharos, Cawl discovered that a shard of the C'tan Zarhulash the Potentate had provided the power source for the ancient Pharos. He forged a deal with the alien star god, in which Zarhulash agreed to destroy the device by removing its heart, creating a miniature singularity.
Cawl seemingly agreed, but this was only a ruse as he actually teleported Zarhulash to a far-off region of the galaxy where the C'tan Shard was surrounded by hostile Necron Tomb Worlds. Zarhulash swore revenge on Cawl for this betrayal.
Cawl was able to escape from Sotha as the Pharos collapsed and gathered much data from an examination of the Pharos' heart, including the location of other Pharos devices and and large deposits of the strategic material known as blackstone across the galaxy.
Cawl hoped to use this information to eventually find a way to build a technological solution that would seal the Great Rift once and for all.
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.