Exocytosis is the fifty-forth short story published in the Horus Heresy Series that was not originally part of a main release anthology. Exocytosis was first published as part of the Black Library's 2016 Advent Calendar, on day number nineteen.
Having long been hounded across the galaxy by the Dark Angels, First Captain Typhon of the Death Guard has limped his fleet to sanctuary in the Segmentum Obscurus -- as a guest of the separatist Luther of Caliban, no less. On the world of Zaramund, long a symbol of brotherhood amongst the Space Marine Legions, Typhon begins to suspect that a new form of corruption has followed him out of the wider war. Will he embrace it, or escape into the darkness once more?
- Exocytosis (Short Story) by James Swallow