Just read the scene where Jaghatai speaks with Magnus's shade.
If only other choices had been made, and Magnus agreed to being restored.... Do we know where Magnus is at this point? In the exchange he mentions he is on a barren rock on the other side of the galaxy.
AND OH HOLY CRAP the tantalizing clue as to the Emperor's name!
One thing I don't get though, is that Jaghatai has this one saying, the one true lie, about Emperors, but in M42 his legion is loyal.... So, this law he applies to emperors does not apply to The Emperor?