Dan Abnett
Traitor General
Gaunt's Ghosts (series) part 8

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.
Warhammer 40k
novel

Also in:
The Lost
Sabbat Worlds: the Digital Collection