The Sigillite (Audio Drama)

The Sigillite is the twelth audio drama for the Horus Heresy Series that was not originally released as part of an anthology or other release. The Sigillite was published online in March of 2013 by the Black Library as an audio CD. The audio drama was later republished in prose format as an e-book in January of 2016 as part of the Tales of the Knights Errant week. The Sigillite was also published as part of The Silent War main series anthology novel.

Official Synopsis
The galaxy is divided, but while his armies prepare for battle the Emperor himself remains curiously absent. In his place stands Malcador - legendary Sigillite, First Lord of Terra and regent to the throne - now arguably the single most powerful man in the Imperium. Imperial Army officer Khalid Hassan reports back to the Palace after a disastrous secret mission into the wastelands of Gyptus, but soon learns an awful truth about the destiny of mankind. Where do Malcador's true loyalties lie?