So, this very handy collection of stories follows the story of Nathaniel Garro and here are some details from The Black Library:
Long has been the road of Nathaniel Garro. After surviving the treachery of Isstvan III and the terror of his escape aboard the Eisenstein, he has become a paragon of loyalty and protector of the innocent. Whilst his Death Guard brothers faltered and fell, his belief in Emperor and Throne remained true. For his charge is a most sacred one, given unto him by none other than Malcador the Sigillite, to recruit the Knights Errant and lay the foundation for a secretive order dedicated to the eradication of Chaos.
It's the whole compelling journey of the fan-favourite character, from his first mission to his discovery of Malcador's true plan. Explore dark corners of the Horus Heresy and see Legions, loyal and traitor, in new ways across more than eight hours of action-packed audio drama.