This is a fairly simple, yet entertaining book: Basically one huge set-piece battle as the Word Bearers tries to get revenge for the stuff at the beginning of The First Heretic, and thus attacks the Ultramarines.
It's basically balls-to-the-wall action, with everything a good 40K arsenal has: Giant robots, spaceships, computer hacking, daemon summoning... The works. It features incredibly improbably stunts (ROBOUTE GUILLIMAN CAN BREATHE IN SPACE!) and simply unequalled amounts of sheer testosterone in that particular way that only 40K can do. It's basically superhumans having a big punch-up. If that's your thing you'll like this, if you expect y'know... Character development, or characters who aren't eleven-foot tall power-armored psychopaths wielding fully-automatic rocket launchers and swords made out of chainsaws, this isn't your thing, there's other stuff that is, even 40K stuff that is. (different from this I mean, not neccessarily your thing, because 40K is an acquired taste)
The battle opens with Betrayal, and there's some genuinely good scenes of shock and confusion, as people who you never thought would betray you (despite you having publicly humiliated them earlier, yeah, no one said Space Marines were bright) do. And then compound the betrayal by kicking every puppy they can see as hard as they can, while crossing every moral line faster than... Something very fast.
It's good (well, this is 40K, so relatively speaking, they're still space commie-nazi-feudal knights) against evil (and note that despite the earlier description of the Ultramarines, there's no moral ambiguity whatsoever, yeah, the Word Bearers are just that horrible people :p)