A rare post-Heresy Primarchs story, Gav Thorpe’s Shadow of the Past takes place on a moon in the Eye of Terror, where Dark Apostle Kalta-Ar of the Word Bearers supervises the building of a grand monument to the dark gods. Protected from the dangers of the warp by ritual means, the Word Bearers nevertheless find themselves under attack by something that bleeds out of the shadows to rend and tear. When they at last bring their monstrous attacker to bear only to find their weapons useless, Kalta-Ar entreats the daemon primarch Lorgar to aid him and his surviving warriors.
While not specified, it’s implied that this takes place not long after the Heresy, in the run up to something of a key moment for the Word Bearers. There’s an interesting stylistic mix of Heresy and 40k, with a plot that cleverly reverses a familiar trope by showing Traitors menaced by a horrifying foe they can’t defeat. There are hints of what’s happening elsewhere, and a cameo from a familiar name, but more than anything this offers a small but surprising glimpse of what the surviving primarchs actually got up to after the Heresy. It’s a Primarchs story about what the primarchs become, and all the more interesting for its unusual nature.
At present Shadow of the Past is only available in the event-only Sons of the Emperor anthology.