To catch the Fallen, you have to become the very evil you were bred to slay.
Show loyalty. Be willing to go deep into the gritty underbelly of the world, where the vilest acts of human carnage thrive and seep into your mind like a black poison, consuming every part of you that was once good. Ignore your dark cravings and keep to the most important rule of all: never save the mortals.
This is the life of a Sentinel.
Some call us the toughest angels in the heavens. Unbreakable. But even the strong can break in the face of temptation.