If you have not read "Vengeful Spirit" by Graham McNeill and you don't want to know a single detail about what's inside the book, then read no further.
Seriously. Stop reading this post.
Because I'm going to spoil an entire sentence from that book for you.
Still reading? You sure you want to? Had enough of my warnings? Good for you.
Vengeful spirit is full to the brim with plot lines that are either carried on from other books in this series, or from the audio dramas about Garro and colleagues. To be fair, the book could have been split in to two books to better tie everything up (and make some of the other characters a bit more, ummm, rounded and less "fallen in to the background", but hey, its a book about Horus and big battles: so you pay money and take a chance if you like the latter).
One line in the book did stand out to me in particular. The Emperor has a wax lyrical moment with other players on Terra and makes the passing comment (paraphrasing here): what chance did my son Alpharius give to my plan?
This is interesting as it indicates that the Emperor simply did not know that Alpharius siding with Horus was part of a wider ploy. Hence although the Emperor is very powerful, he has strong limits and really doesn't see things that one might imagine he might intuitively (or psychically) be able to discern. Equally, he was a tiny bit distracted at this point in the chronology due to Magnus having smashed the wards around the Terra web way gate. So, forgivable, perhaps.
If only the Emperor and Malcador knew that the Alpha Legion were secretly loyal. That could have shaken things up by a good margin...