There will be a point in the Vestibule prologue near the end where you will have to execute 3 torture attacks before a timer runs out. When you attack enemies you gain orbs, which are displayed under your health meter. When this meter is full, you will be prompted to push
together. This will execute a torture attack if you're next to an enemy. This achievement will be obtained before you go progress past the prologue because you need to execute 3 torture attacks before the timer ends to get past this battle in the prologue and move on. Once you execute the first one, you'll get the achievement.