During your first visit to the Martyr you will rescue a critically injured Adeptus Astartes named Caius Thorn. From Thorn, and through clues discovered on the Martyr, you will learn of another Inquisitor present somewhere on the Martyr named Klosterheim. About a third of the way into the Season 1 campaign you will eventually catch up with him. You will fight alongside him for a few missions, and then eventually confront him regarding the Alpha Pariah, at which point you will be given a Morality choice.
Choose the Puritan path choice to unlock this achievement.
If you choose the Radical path choice, you will unlock this achievement instead:
If you like, you can unlock both achievements in one playthrough. Simply dashboard out after the achievement pops (do NOT complete the mission), quit the game and restart, then play through the mission again, making the other choice this time. The mission is not that long, however, you need to play through the campaign at least twice for a number of other achievements anyway, so it's up to you.