Another short story written by Aaron Dembski-Bowden, this one starts with Lorgar's Word Bearers and Angron's World Eaters facing each other in the void, on the edge of starting a battle between their fleets. The story can be found either as a part of the "Legacies of Betrayal" anthology as a book or book/eBook on Amazon or on its own as an audiobook on Amazon or at the Black Library.
The only thing stopping Lorgar and Angron from fighting each other is the sudden appearance of a Xenos (Eldar) fleet bound on killing Angron and stopping him, in their own words, short of becoming "the son of the Blood God".
Other than that we don't get much apart from a duel/conversation between Khârn and Argel Tal, who has already become possessed, and a quite nice demonstration of the World Eaters' rage attitude when facing conflict. Oh! And the unforgettable moment when Lotara Sarrin gives the order to fire the Ursus Claws. Yes, that one: Fire the Ursus claws!
Next in line is "Heart of the Conqueror", and then "Betrayer". But before "Betrayer", following some advice I saw when looking for info on the World Eater's novels reading order, it's time for "The First Heretic" and getting to know the ins and outs of the Word Bearers' turn to chaos. I have already read both stories, and let me say that "The First Heretic" is absolutely awesome. BTW Argel Tal happens to experience possession in "The First Heretic", so it would be read BEFORE Butcher's Nails.