Dark Heart is a short story published in The Horus Heresy series that was originally part of an anthology. Dark Heart was first published as part of the Mark of Calth main series anthology in April 2013. The cover image used is that of the e-book version Dark Heart which was republished separately.
When the Word Bearers launched their surprise assault on Calth, it marked the beginning of their righteous campaign of vengeance against the hated Ultramarines Legion. But for one young Acolyte of Kor Phaeron, it is not the sons of Roboute Guilliman that he seeks to bring low -- through infernal pacts and daemonic power he strives to carve out a destiny for himself in the midst of the greatest war that the galaxy has ever seen. The name of Marduk shall be spoken with awe for millennia still to come...
- Dark Heart (Short Story) by Anthony Reynolds