The Purity of Ignorance(2016)

John French

While the Space Marines and the Imperial Guard fight the myriad alien enemies that seek to overwhelm mankind, the Inquisition pursues a different kind of war. For theirs is a war fought in the dark places of man’s soul – a battle against the corruption of Chaos, a battle for the survival of humanity. Inquisitor Covenant must hunt down the daemon at the heart of a planetary conspiracy, but how high a price will he and his team pay to defeat the evil, and are they even aware what the cost of success will be?

Read it because
Though he was introduced to the Warhammer 40,000 universe at the same time as Eisenhorn, there's never been fiction about Inquisitor Covenant... until now!