Armour of Faith (Space Marine Battles Novella)

Sent to secure a seemingly unimportant fortress on the war-wracked world of Orath, Ultramarines Chaplain Sentina and his small squad of Centurions find themselves at the centre of a plot to open a new Eye of Terror in the heart of the Imperium. As daemonic hordes assail the fortress and a fell power rises within, it falls to Sentina to unravel the design of the Ruinous Powers and prevent Chaos from consuming an entire sector of space… whatever the cost to his life or even his soul.

Read it because
The dramatic conclusion to the Plagues of Orath trilogy. A squad of Centurions and a Chaplain fight their way through tides of daemons to the heart of the mystery on Orath. The Ultramarines’ humanitarian ideals come up against the need to win the war, and Chaplain Sentina fights a personal battle against his inner daemons even as he battles the inhuman hordes of Chaos.