Warhammer 40k - Space Marines Battles Novella - Armour of Faith by Graeme Lyon

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.