Burden of Duty is the seventh audio drama for the Horus Heresy Series that was not originally released as part of an anthology or other release. Burden of Duty was released as an MP3 format download in October 2012 and was written by James Swallow. It was later re-released as an audio CD alongside the story Grey Angel, but can still be bought as a stand-alone audio drama MP3. The story was also published as part of the Garro main series anthology novel.