The Serpent Beneath (The Horus Heresy #Novella)

When your entire existence is based around deceit, who can you trust? That's the question that plagues Omegon, one of the twin primarchs of the Alpha Legion, as he prepares a team of elite operatives for the most difficult and dangerous mission any of the Alpha Legion have ever attempted to infiltrate a secret installation controlled by the Alpha Legion itself!

Read it because
It's as much of an insight into the secretive practices of the Alpha Legion and the differing agendas of their shadowy leaders as you're likely to get... assuming that you can trust any of it, of course!