I just finished Age of Darkness, a collection of Horus Heresy stories. I don’t generally like short stories, and one cavil I have about the Horus Heresy novels is the difficulty of getting to know totally new characters with funny names, all of whom end up dead in the end. At least over a 300-page novel the initial investment tends to pay off; in a 30-page short story it could be an irritant.
The stories are, however, generally very good. They are all set in the war between the traitor and loyalist legions, when massive violence is simply the cosmic norm. One could see it as akin to the Peloponnesian War in space, a conflict from which no place or people is safe. Thucydides could have written an interesting Horus Heresy novel, perhaps.