A Deus Ex machina explaining how Guilliman manages to bring in Sanguinius, for whatever reason, since he ends up at Terra anyway. And for whatever reason, they bring the Lion into it, though I guess it was never truly explained why he wasn't at Terra? Never read any of the Dark Angels books that weren't in the HH series. And then asshole Curze, who they've been trying to interject for like 4 books now. It's okay until Abnett gets pulled into the Vulkan Lives bullshit. Yeah, Vulkan's immortal and Curze drove him crazy. He can't die and Curze is pissed that they keep fighting and he can't kill him. But it doesn't matter, because Curze gets away anyway. And ends up on Macragge just like he did in the beginning of the book, despite being warped away towards the end. Pointless, really. Another Deus Ex to make him conveniently go away for a bit.
The Lion and Guilliman make up and make out, and in the last ten pages or so Sanguinius shows up, they agree to make the second Imperium in the case that Terra has already fallen and make Sanguinius the new Regent (or Emperor, if the big E is dead), since none of them can find the Astronomicon.
What I can't get is that the Lion and Sanginius can't see the Astronomicon, although the Khan, Leman Russ, and Corax were able to find it, as well as the Imperial Fists. Lorewise, at least within the HH series, it makes sense that the Ultrasmurfs can't see it because of Lorgar's Ruinstorm, but the Lion and Sanguinius weren't in Ultramar when that happened. Why can't they see it, when every other loyalist legion can? I suppose they'll Deus Ex that too.