Cypher: Guardian of Order (Black Library Advent Calendar 2013 #25)

For a select few amongst the First Legion, the Northwilds of Caliban hold a particularly dark secret. At the command of Luther himself, young Librarian Zahariel accompanies the Lord Cypher on one of his mysterious forays into the unknown depths. But an ancient evil is stirring once more and, with rumours of galactic civil war creeping back to their home world, it may have already sunken its claws into the Dark Angels Legion...

A Horus Heresy audio drama by Gav Thorpe