Warmaster is an audio drama by John French. It was published online in December 2012 as part of the Black Library Advent Calendar (2012) and later republished in prose form in the Legacies of Betrayal anthology. The audio version was performed by Ramon Tikaram, Toby Longworth and Tim Treloar.
In the strategium of the Vengeful Spirit, Warmaster Horus appears to talk to himself as he ponders his invasion of the Imperium. He expresses regret that, despite the righteousness of his cause, only the worst of the primarchs have joined him, describing them as broken monsters with which he must remake the galaxy. He wishes that Roboute Guilliman was on his side and compares Alpharius unfavourably with Corax. At the end of his speech he is revealed to be addressing the skull of Ferrus Manus.