The Abyssal Edge (The Horus Heresy: Primarchs)

A Horus Heresy story
An accidental discovery puts an archivist in danger when he finds evidence of a time that the the Night Lords and Thousand Sons almost went to war against one another…READ IT BECAUSE
Aaron Dembski-Bowden crafts a tale that looks at a pair of primarchs through new eyes, revealing them in all their majesty, glory… and terror.THE STORY
When an archivist uncovers an unthinkable truth, he comes to the attention of the First Captain of the Night Lords, the dreaded ""Prince of Crows"" himself. Terrified yet enrapt, Ulatal listens as Sevatar tells him a tale that cannot exist in any Imperial archive. It is a tale from the world of Zoah, of the moment when Konrad Curze and Magnus the Red almost went to war...

This story is also available in the Sons of the Emperor anthology.