I'm not sure about the exact plot as I haven't read it in detail. But generally, in Dark Angels novels, there are often themes of the Dark Angels' history, their secrets, and their battles in the Warhammer 40,000 universe.
Well, without actually reading it specifically, it could be about the origin of the Primaris. Maybe it details their creation, their first missions, or the challenges they faced when they were first introduced into the universe of the story.
One main theme could be secrecy. The Dark Angels are known for their hidden pasts and secrets, and this novel might explore how those secrets are affecting them. Another could be loyalty, as they have to decide who or what they are loyal to in the face of this splintering darkness.
As I haven't delved into 'Darkness Devours a Dark Angels Novel', it's difficult to say exactly who the main characters are. In Dark Angels stories, there are often leaders, librarians, and veteran warriors. It's likely that the main characters in this novel would be from among these types of Dark Angels. They could be on a quest to uncover a mystery or fighting against a great threat that is related to the 'darkness' mentioned in the title.
The main characters could be a group of Primaris who are on a special mission. Each of them might have different personalities and skills. One could be a strong fighter, another a strategist, and there could be a tech - savvy Primaris as well. They would have to work together to achieve their goal in the story, facing various obstacles and enemies along the way.
Well, it's likely a novel centered around the Dark Angels. It probably delves deep into their lore, their battles, and the internal struggles within the chapter. Maybe it showcases their unique combat styles and the dark secrets they might be hiding.