Grey Angel is the sixth audio drama for the Horus Heresy Series that was not originally released as part of an anthology or other release. Grey Angel was released as an MP3 format download on August 3, 2012 and was written by John French. It was later re-released as an audio CD alongside the story Burden of Duty, but can still be bought as a stand-alone audio drama MP3. This story was later re-published in prose format in January 2016 as part of the Black Library's "Tales of the Knights Errant" week. The story was also published as part of The Silent War main series anthology novel. The title received updated cover artwork when it was released as an e-book, both the original cover and the updated cover are shown.