So what do we know about the oft-mentioned Adranti and their heresy?
There are some tantalizing hints about the Adranti people contained in the adventures from Purge the Unclean:
1.) There were a human civilization not connected to the Imperium of Man, that probably got lost during the Age of Strife.
2.) According to the official timeline ( http://www.fantasyflightgames.com/dark-heresy/pdf/timeline.pdf ), the Adranti people were eradicated by the Adeptus Mechanicus: "In recognition of their invaluable assistance and heavy losses in the purging of the taint of the Adrantians, as well as their service to the Crusade in decades past, Drusus grants the Lathe system to be the sole domain of the Adeptus Mechanicus in perpetuity, and ratifies their claims to several other worlds and rights of free and unchecked passage through the stars the Crusade has conquered."
3.) The "Xenos" artifacts from Illumination and Rejoice for You Are True were, in fact, of Adrantian manufacture. The first artifact could somehow interrupt psychic abilities, while the second one was an auspex / scanner used to find potential psykers.
4.) They are connected to the Serrated Query. The Faceless One is probably of Adrantian origin?
5.) There is an Adrantis sub-sector (containing the Adrantis Nebula) in the Calixis Sector. It is described with a single sentence: "This subsector of the Calixis Sector is the home Magos Genetus Halix Redole, a famous researcher dedicated to eradicating Adranti genetic corruption."
So, what else do we know about the Adrantians?
PS I copied this question from another forum. Thanks to Deathstalker for summarizing what we know.