Poor Horus. He had to destroy about a third of four of the Legions that would follow him in his Heresy. Then he had to lose even more Space Marines because Angron felt the need to go melee against the survivors on Isstvan III. Then he had to deal with the Night Lords falling apart and splitting into who knows how many warbands. To say nothing of the increasing unreliability and degeneration of the World Eaters. Now one of the smallest Legions under his command ...
No wonder things fell apart for Horus in the end.
in all seriousness, it sounds like a story for the sake of telling a story. The compliment I paid to Dembski-Bowden on his Facebook page for the Abaddon piece he wrote was that it avoided being exactly that.
He managed to squeeze nuance and impact in a tiny story and made what was ostensibly a plug for a novel feel like anything but.
By contrast, I can't help but feel that this is an attempt to make up for the handling of Lucius in Angel Exterminatus.
Frankly, I felt like that was one of the weak points of the novel; it didn't really serve any purpose and it went against the grain of Lucius being this amazing prodigy. So what, now we get a short story that emphasizes what a badass this guy is? We knew that before a half fleshed-out character that featured less than almost any one else in the supporting cast (but happened to be a supreme swordsman as well) cut him down to size.