Age of Darkness is an anthology of Horus Heresy short stories, most of which aren’t that great. From a distance, all the books are about Space Marines shooting each other in the face, but each of the interesting novels is more interesting for the things that happen around the battles, rather than the battles themselves. The shorter stories, without the room to breathe, have much less appeal.
Also, there’s stuff about Dark Angels, which are always dreck, even when Aaron Dembski-Bowen writes about them (a grinding story about people punching each other in the face in a ruined crater of a world), but there was one highlight, set in the fortress of Schadenhold (because, just like schadenfreude, there’s pleasure in watching bad things happen to people that attack the fortress). Other than that, a forgettable read – nothing that really stuck out for me to remember, even a few hours later.