"Descent of Angels" attempts to tell the tale of the origins of the Dark Angels - maybe. What it really tells is the origin of Squire, turned Knight, turned Aspirant, turned Space Marine, turned Librarian Zarahiel, who by the end of the novel, becomes relegated to a near non-entity, apparently. Lion el'Jonson comes across as one of the least impressive Primarchs at this point. People aren't falling to their knees in his presense, or becoming rendered speechless as they find themselves in the presence of a living Demi-God. Which comes across as especially strange on a world which is portrayed as medieval (with the exception of bolt pistols, chain swords, and power armor) where one would think groveling to a go-like being would be second nature. The Lion is just, you know, a tall, impressive guy, that people look up to. He lacks the presence, power, and charisma of the other Primarchs that we have met thus far. I also think that Luther would be prime for a main character treatment, with all of his inner conflict and turmoil, but he's just there, as well...becoming vitally important for a second, and then banished to the background yet again.
Overall, I feel that this was a somewhat underwhelming story for the Horus Heresy series and it feels like it's missing plot elements to tell a complete narrative. Admittedly, yes, this is a #6 in a rather lengthy series, but up to this point each entry has been rather self-contained with the next novel building upon an already established plot - or filling in backstory. This did none of that. Nothing in here touches upon anything else previously seen in the Horus Heresy, with the possible exception of The Watchers, who claim to be part of a Cabal that seem to be attempting to stop Horus (which I think may be touched upon a bit more in the next entry, "Legion").
I realize in a long-running, 50-plus novel series, that everything can't be amazing. But for a Chapter as storied as the Dark Angels, with their very ambiguous background (are they or aren't they heretics? what are they hiding?), this is somewhat meh.