Astropaths

Astropaths are chosen from the multitudes of psykers brought to Holy Terra on the black ships due to their great power. Once chosen, Astropaths are spiritually joined to the Emperor. This is why only powerful psykers can become astropaths, as such an experience can be fatal. Many do die, while most survivors lose their eyesight.

Being joined with the Emperor is necessary however, as it greatly heightens Astropaths' already formidable powers and gives them the ability for which they were chosen - to send psychic messages through space across large distances, thereby becoming the main network of long distance messaging in the Imperium. Most ships have an Astropath on board and most planetary governments have access to Astropaths.

Without the Astropaths' unique ability, long-distance communication in the Imperium would be impossible. However their powers are still susceptible to the local conditions of the Warp (such as warp storms) and as such, their messages can be greatly delayed or lost. Due to the strenuous nature of their job many Astropaths are physically frail and can die quite young through exhaustion