User blog comment:Boradzombiecreeper4/Blood Raven origins/@comment-31.49.62.242-20140807162730

While direct in-universe mentions of a connection between the Blood Ravens and the Thousand Sons is scant, a significant amount of allusion is made to the subject in a variety of sources. A summation of these incidences and their implications follows:
 * The Blood Ravens are established to have enough similarities in battle doctrine (i.e. high number of psykers and the use thereof) to the Thousand Sons that it has been noted by observers within the fiction itself.2b
 * The pre-Heresy heraldry of the Thousand Sons is similar to the heraldry of the Blood Ravens; specifically the Thousand Sons Corvidae cult, who as well as wearing the red armour of all Thousand Sons, also sport a black raven insignia (though admittedly of just the raven's head rather than the whole bird).12a
 * The psyker Kallista Eris had a vision concerning the fate of the Thousand Sons shortly before the Burning of Prospero. Among other things she mentioned "The Ravens. I see them too. The lost sons and a Raven of blood. They cry out for salvation and knowledge, but it is denied!"12b There are several possible interpretations of these words, but the language used is clearly pertinent to Blood Ravens speculation.
 * The Blood Ravens' motto "Knowledge is power" was uttered by the Thousand Son Corvidae member Revuel Arvida upon his decision to dedicate himself to discovering the true causes of his Legion's destruction and combat them. Arvida was a member of the fleet-elements of the Thousand Sons who were sent into hiding by Magnus the Red prior to the Space Wolves' attack upon the Legion and therefore did not fall to Chaos during the Burning of Prospero.10, 12c Thus we have a non-Chaos aligned Corvidae Son dedicating himself to fighting the Ruinous Powers in the future. It is also worth mentioning that his name, Revuel Arvida, has certain stylistic similarities to that of Azariah Vidya, Great Father of the Blood Ravens. This motto was also stated by the Thousand Son Mhotep whilst locked in combat with a daemon.13a A counter-point to this potential link is that the Blood Ravens motto is a tenet uttered by various, non-Blood Raven individuals in a variety of other sources.
 * The Blood Ravens' Rahe's Paradise outpost was discovered to be built upon the ruins of a previous Astartes structure, one that dated from "ten millennia ago". While no identification markings could be found upon it, a red-painted Astartes shoulder plate was excavated from these ruins. Additionally, these ancient Astartes were revealed to have allied themselves with Eldar forces present upon the planet to defeat a great evil, and to have agreed to maintain stewardship over the world in order to thwart this evil should it return.8c Ten millenia ago references the birth of the Imperium, although the Thousand Sons were not the only Astartes Legion to wear red as their primary colour at this time, seeing as the Blood Angels continue to do so (and one cannot dismiss the notion that this may include the two unknown Legions).
 * During an incident on the planet Arcadia, Librarian Rhamah of the Blood Ravens suffered amnesia and fell in with the Prodigal Sons warband of Thousand Sons sorcerer-lord Ahzek Ahriman. Throughout this association Ahriman and his warband referred to Rhamah variously as "Son of Ahriman", "friend of Ahriman" and as a "lost brother". Furthermore, when Rhamah locates an ancient Eldar tome - called the Un-Founding - that bears the crest of the Blood Ravens upon its cover, Ahriman concedes that he once owned a copy of the book and that another one was in the possession of Azariah Vidya (of whom Ahriman states, "I knew Vidya better than you might expect"). He further states to Rhamah "We are not so different, you and I. We were not always so different...there was a time, long ago, before the Change, when the Thousand Sons of Magnus wore the blood-red armour of their Primarch."8d No explanation is given for what any of this means, and it should be noted that Ahriman is a master of deceit and trickery who was attempting to use Rhamah for his own ends.