
The cast. ADB's trademark sympathetic Chaos Marines, but riven with a hell of a lot more tensions than even Talos's crew and lots of foreshadowing about how the gang breaks up over time. The book's worth it just to watch them interact.
The Anamnesis, and the explanation for machine-spirits in general. (Finally!) And now I'm wondering just what the standards are. Is a "smart" bolter shell sufficiently holy if a Techmarine jams a brain cell into the tip? Does Mars stick to the standard as strictly as they're saying? Loads of potential here.
Abaddon. The grand failure of Chaos is charming, charismatic, intelligent... nothing like his appearance in the Horus Heresy. (Where, thematically, I suppose he's still a temperamental child. This is Abaddon grown up.) I don't think we'll ever get an insight into how his mind works now, but he's fascinating to read about. Especially his science projects. Wouldn't have thought him capable of such things.
The throw-away lines about what the Black Legion will be getting up to. Even if ADB never finishes the series, the potential to see Abaddon humbling at least six Daemon-Primarchs will keep me reading.
Primogenitor Fabius. Honestly he's suffering from the full Worf effect here, but it's muted somewhat by...
Spoiler: Horus freaking Lupercal, apparently fully restored.
I know this is a spoiler-free zone but that's too big to put out in the open on the first post. Of all the ways I expected the book to end, that was nowhere on the list.
Spoiler: It raises a lot of questions about the Primarchs in general and Horus in particular. We know they're part ethereal, and it seems that the clone Primarchs share consciousness with their originals. The fact that Horus recognizes Abaddon makes me wonder how much of Horus is left, at least in potentia. Could it be that the Emperor dissipated him so badly that he can't reincarnate without a vessel? And with a vessel, could Horus, the original Horus, potentially return?
Not that it matters, because Abaddon skewered him, with the best line ever: "I am not your son." Which could be read two ways: either Abaddon doesn't accept clone-Horus as Horus, or Abaddon would say the exact same thing to the read Horus.
And likely more I can't think of right now, but I'll end with Khayon's declaration of the End Times to his audience. Could it be that the Warhammer 40,000 universe is approaching even closer to the end of itself, a la Fantasy? Or is ADB teasing us a bit?