Astronomican - Horus Heresy: During the Age of Technology before the Astronomican existed, its functions were fulfilled by a techno-arcana device known as a Pharos. One of these xenos devices, later determined to be of Necron origin, was discovered beneath Mount Pharos on the world of Sotha by the Ultramarines Legion during the opening days of the Horus Heresy.The Pharos was part of an ancient interstellar navigation system, both a Warp beacon and route-finder, and it also permitted instantaneous communication across unimaginable distances.Unlike Imperial Warp-Drive technology that used the Immaterium to by-pass realspace, the quantum function of the Pharos once allowed for site-to-site teleportation, perhaps through a network of gateways. Its fundamental function lay not in the utilisation of psychic energy, but with empathic power.It was believed that other devices like the Pharos still existed as way stations, or once did, on other worlds throughout the galaxy. Yet the Pharos technology did not act as a single beacon visible across the galaxy like the Astronomican.It was theorised that the Pharos and other stations like it were once used to create a network of navigational pathways between the stars, as opposed to a single, range-finding point like the Astronomican. In that sense, the Pharos was more a lantern than a beacon. The Pharos could be tuned and pointed, illuminating a site or location for the benefit of range-finding.Following the Battle of Calth, Erebus of the traitorous Word Bearers Legion had managed to complete a blasphemous ritual on Calth's surface, which summoned a Ruinstorm to the galaxy's Eastern Fringe -- a monstrous Warp Storm larger and more destructive than anything space-faring Humanity had witnessed since the days of the Age of Strife. It split the void asunder, dividing the galaxy in two and potentially rendering vast tracts of the Imperium impassable for centuries.Utilising the strange abilities of the Pharos, the Ultramarines were able to illuminate Macragge, lighting it up as a psychic bright spot that was visible throughout realspace and the Warp. Despite the effects of the Ruinstorm, the Pharos provided a directional beacon for Warp navigation in the absence of the Astronomican.Though Primarch Roboute Guilliman was loathe to use xenos technology, Ultramar had to be held together, and to rebuild the Five Hundred Worlds, the Loyalists had to restore communication and travel links by piercing and banishing the age of darkness initiated by the Traitors.Just before the outbreak of the Horus Heresy in the early 31st Millennium, the Emperor of Mankind's Imperial Webway Project had required more and more of the Emperor's psychic might as He sought to penetrate the corridors of the Aeldari's Webway and claim it for Mankind, a task that dangerously weakened the Astronomican's signal across the galaxy.Malcador the Sigillite suggested a compromise: the beacon would still be guided by the Emperor Himself, but a large part of the required energy would be provided instead by the psychic and life energies of 10,000 sacrificial psykers, allowing the Emperor to pour His own psychic might into finishing His great project.The Emperor approved the notion only with the greatest of reluctance, and also made it clear that it would be only a temporary solution, for He had no intention of claiming the lives of thousands of innocent men and women on a regular basis, indefinitely.Tragically, the Emperor would never resume His former role as the active ruler of the Imperium because of the mortal wounds He suffered at the hands of Horus during the Horus Heresy's climactic Siege of Terra. Although His mind would continue to calibrate the psychic beam's frequency following His physical internment in the Golden Throne, the Emperor was never able to resume powering the beacon solely by Himself, and the once-temporary measure of having the Astronomican powered by the regular sacrifice of thousands of psykers became permanent.At the present time, in the 41st Millennium, the Astronomican is a complex piece of machinery powered by 10,000 specially-trained psykers, whose life-forces are exhausted in a matter of solar months maintaining the beacon. Their ranks must therefore be constantly replenished with fresh recruits drawn from the tithe of psykers brought by the Black Ships to Terra to be used by the Adeptus Astronomica.