Chief Astropath Xiao is the Senior Astropath in the Adeptus Astra Telepathica of the Calixis Sector in the Segmentum Obscurus. Xiao is an improbably tall and broad-shouldered man with a shaven head and a long, grey-streaked beard. His blindness - he wears an embroidered band around his head to cover the empty eye sockets that are common for soul-bound Astropaths - is the only note of physical weakness about him, for even in his robes he is obviously a large and muscular man.
Xiao does not permit anyone to send a message through his astropaths on the Calixis Sector's capital world of Scintilla unless he deems it necessary. Lord Marius Hax, the Sector Governor of Calixis and Planetary Governor of Scintilla, is rarely refused by Xiao, but anyone else must justify the use of the astropath's rare talents. Though Xiao is unfailingly polite, he is still a very intimidating man and calmly refuses to compromise once he has made his decision. Xiao is one of the most powerful psykers in the Calixis Sector but he never sends astropathic messages personally, even for Hax, always having one of his astropaths do so instead, although it is rumoured that he has an arrangement with someone very powerful for whom he sends messages exclusively.
Senior Astropath Xiao dwells in the Bastion Porphyr, the tallest tower of Hive Sibellus on Scintilla. The Bastion Porphyr houses some of the Calixis Sector's most powerful astropaths, all members of the Imperium's Adeptus Astra Telepathica.
Just as Warp travel within the Imperium of Man is a complicated and inexact science, so too is the business of exchanging interstellar messages between the many and varied planets and star systems that make up the Emperor's realm. Radio or optically-based planetary communications systems such as vox-casters, hardwired lines and the more advanced vox-communicators suffice to pass messages amongst the nations or cities of a world, yet have almost no use beyond the bounds of the planet's surface. Such devices, their radio or light waves restricted to the speed of light, require many years for their signal to reach even the nearest planet and have no surety of even being detected when they arrive.
The perils of travel through the Warp ensure that human or Servitor messengers are just as unreliable and potentially as slow as radio or other electromagnetic wave-based communications. The Imperium, then, is forced to rely upon communication by psychic, or astropathic means. Astropaths communicate with symbols and iconic images, projecting these messages instantaneously through vast distances of space by means of psychic power, which broadcasts through the non-Euclidean dimension that is the Immaterium.
This process is usually exhausting and requires ritual and focus in order to keep the pskyer in the right frame of mind. These focusing efforts can take a wide variety of forms, such as use of the Emperor's Tarot, vision quests, automatic writing, trances, séances and the like. The Gaolist Astropaths of Hredin, for example, spend many years etching their messages onto painstakingly illuminated sheets of iron and then destroy the work of art upon a massive grinding wheel. The pain of annihilating a much-loved labour is said to produce psychic messages of unparalleled clarity.
- Warhammer 40,000: Dark Heresy Rulebook pg. 294