Having finally got my hands on Inferno and reading the Space Wolves section there is mention of the Trefoil of legions which were formed in greater secrecy than the rest. Those being the aforementioned Wolves, as well as the Salamanders and Alpha Legion.
The need for secrecy with the Wolves seems plainly obvious, they were the executioners, designed and trained to kill/destroy whoever the Emperor decrees, even if that is other legions or members of humanity.
The Alpha Legion are a legion all about secrecy so why should their creation be any different? Although I question what about their founding required an extra level of secrecy. Based on the Wolves secrecy it is likely because it was some sort of end game requirement that the Emperor needed the Alpha Legion for that other legions couldn't know about. So the only thing I can think of is that they were maybe to be designed as a secret police either for the purpose of infiltrating the other legions and keep the Emperor abridged of any dissent or problems that could need addressing by the "Executioners". Or they were to be the secret police across the galaxy, using their ability to recruit human operatives to keep the human populace in line.
So two of the three make some arguable sense for their secrecy, but what was the Salamanders legion job that required that extra level over the others? Can understand if Vulkan needed extra secrecy what with his Perpetual nature and all. But that trait doesn't seem to have exactly transferred to his sons. So what about the legions foundation required them to be kept secret from the others? Does anyone have any opinions to the idea?
Or other opinions on the Alpha Legion?