Dark Heart, the Horus Heresy story that I wrote for the forthcoming anthology Mark of Calth is now available to download here.
The story is set during the Word Bearers' attack on Calth, and is designed to tie in with Dan Abnett's awesome Know No Fear. It features a certain young Word Bearer called Marduk taking part in the attack on the Zetsun Verid Yard - and the subsequent counter-attack against the Word Bearers led by the big man in blue himself...
I've also written a short blog about writing this story for the Black Library site - check it out.