Originally Posted by
Wraith
I'm pretty sure I can make an educated guess.
Super-duper Deliverance Lost spoilers...That you probably guessed just by reading the back of the book.
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Due to the edict of Nikea (psykers no psyking!), Alpha Legion are able to infiltrate multiple Legions. Due to entire Chapters of Legions being separated during the Istvaan massacre, Alpha Legion basically insert themselves into the Raven Guard Legion. It's unclear how they actually do this...But it's handwaved along that Alpha Legion still use psychic powers to implant dead or dying Raven Guard personalities, memories and facial features onto Alpha Legionaries.
We're finally given a very brief example of what happens when a Marine eats something to gain its memories.
Alpha Legion 'Raven Guard' are taken by Corax to Terra. And we're not told until very late in the book who the Infiltrators are (I'll leave that part spoiler-free). Since it's all told from their point of view, and they all refer to themselves as Alpharius...Which does get really confusing, but, on some level, I can see how that may be the point...
Where they find out what Corax has been given, they simultaneously set out to wreck it and take it. Obviously if the formula was in the hands of someone...Say...That Emperor's Children fellow, Apothecary Fabius...The heretic side of the war could do great things...
Wait...None of that is in Lexicanum. Gav Thorpe is obscure. Nevermind. None of it happened. Right? Right?
EDIT: I think the interesting part about a lot of the Loyalist side books, is how pretty much all of them, at some point, make an issue of bringing back psykers, regardless of what the Emperor says.