Ok, I don't really plan to give too much away, but if you want to approach this book with absolute mystery, don't read this post.
First off, this is not the culmination of the Dark Angels saga. I had thought it would be and had been eagerly anticipating the answers. Those will have to wait for another book I'm afraid.
Yet again they have left doubts about both the Lion and Luther. Right now Luther is looking the more suspicious, but I think that is a ruse to keep the Dark Angel players spirits up. Both are shown to be exemplary heroes, but both also have their dark sides. In particular with The Lion, I think I can see how he could fall - pure ambition. At the end of the book he is being ambitious for the future, restarting the Great Crusade once Horus has been defeated. I wonder if the Istvaan V massacre will perhaps make him think that he is preparing for a future with the wrong side... and might be open to a switch if the offer is right.
Luther also shows a dark side. But it's far more open and obvious... hence why I think it's a ruse. He is also a hero, though a flawed one.
The Watchers in the Dark make another appearance, and reveal some more info. They also imply they may be part of the Cabal mentioned in Legion. Nothing solid... but I suspect they are part of it.
We are also introduced to (I believe) Cypher, as in THE Cypher we all know and love today. He himself is an enigma. He has a shady (though well educated) past. He is knowingly instated to his position of authority by The Lion... seemingly to be loyal to him above all else. Yet he then seems to develop an excellent relationship with Luthor. It's a mystery... I reckon he encountered the watchers early in his life and still acts under the guidance of the Cabal to this day.
I'm disappointed I didn't get the answers I'd been hoping for from this book, but it was a good book. To move the timeline along quicker would have meant less action, and that would have been sorely missed. The answers I did get were clever - teasing you along but not committing either way till they're good and ready. Like most horus heresy novels. So with another Dark Angels one to go, Space Wolves and Thousand Sons, and I think they just HAVE to do Night Lords at some point... that's 4 books before we even reach Terra! And surely sometime they must explain the background of that bloody swine Erebus! Ah the Horus Heresy novels - I want the answers they will provide... but I don't want them to end! Sooner or later that impasse will be resolved. I just wish they'd churn them out a bit quicker - I read both Descent of Angels and Fallen Angel at work the last 2 days. Sadly GW churn them out at a speed more suited to those gits who take months to read a book (if how long mine go on loan for is any indication :oP)
A good book. But WHEN is the next one out! No, not the next Heresy one, I know and am looking forward to that. I want to know when the Dark Angels issue will finally be resolved and I'll finally have proof to wave in Richs face while I laugh at his Italian Primarch!