Quote Originally Posted by Nabterayl View Post
(for those who don't know, every sanctioned psyker in the Imperium, from the most potent Grey Knight grand master to the lowliest astropath, undergoes a ritual called the "soulbinding" in the Presence of the Emperor, after which the psyker has a resistance to the Warp that he did not have before; many also receive visions during the ritual).
This is not exactly true - only those psykers too weak to withstand the warp on their own are soulbound. Soul-bound psykers become astropaths. The soul-binding ritual destroys the optic nerves and often other nerves as well and so all astropaths are blind and many are deaf or lack the senses of smell or touch.

"...only a tiny proportion of the very best are judged strong
enough to survive without some form of psychic protection. So psykers
can be found throughout the Adepeus Terra, the Inquisition, the
Legiones Astartes, the army and the fleet. However, over 90% of
psykers in imperial service are members of the Adeptus Astra
Telepathica, known commonly as Astropaths. Astropaths are created
from those who have considerable powers. but who are not
individually strong enough to withstand the attentions of psychically
attuned warpcreatures. Were they simply allowed to develop without
interference they would quickly find themselves in serious trouble
endangering the rest of humanity as well as themselves. However
the Imperium has a use for them. An Astropath is an Astro-telepath,
an individual capable of communicating with others of his kind over
vast interstellar distances. In the Age of the Imperium, where worlds
are lightyears apart, this is the only practical means of communication.
for this reason the network of Astropaths is very important to the
Imperium, and every spacecraft, research station, outpost, etc., has
its own Astropath. Even small planets need hundreds of these useful
servants, while large worlds may have thousands and Earth itself is
home to tens of thousands.
All Astropaths undergo a special process which moulds their powers
and at the same time strengthens them against psychic danger. This
is called the soul-binding ritual and only the Emperor has the power
to perform it. It takes place in the great palace where the psykers
are led before the Emperor one hundred at a time. Knelt before the
Emperor they must endure several hours of agony whilst the Emperor
uses his powers to reshape their minds - mingling a little of his
immeasurable power with their own. Unfortunately, the Emperor's
mind is so powerful that not all candidates survive the ritual. Some
are driven insane. and all have their personalities altered to some
extent. The raw energy of the Emperor's will also has another effect.
So powerful are the forces involved that many of the more deIicate
nerves can be damaged, especially the optic nerves. Consequently all
Astropaths are blind, whist many may also lack any sense of smell, touch or hearing.


pp147-8 WH40K Rogue Trader