Traitor General: Gaunt's Ghosts: Warhammer 40,000, Book 8 (Unabridged‪)‬

Book 8 in the Gaunt's Ghosts series.

Infilitrating enemy-held Gereon, Gaunt's Ghosts must deal with daemonic killers, reluctant allies and the taint of Chaos itself. Their mission: to find and stop a traitor general. 

Listen to it because:

Gaunt's Ghosts, as a handful of troopers - including Gaunt himself - embark on a deadly mission into the heart of enemy space...a mission that Gaunt has personal reasons for wanting to finish. It's a deadly journey into the darkness of Chaos that will change the Tanith forever.

The story:

An Imperial general has defected to the enemy, and the secrets he carries could spell the end of the Sabbat Worlds Crusade. Colonel-Commissar Gaunt and a small team from the Tanith First must infiltrate an enemy-held world and gain the trust of the local underground, all the while trying desperately to avoid the ever-present taint of Chaos. Their mission: to stop the traitor general from revealing his secrets - by any means necessary.

Written by Dan Abnett. Narrated by Toby Longworth.