For this achievement you need to complete all 3 anima events. The first one being south of the pit stop in a house in chapter 3 . The 2nd one is in southern business district in jukes diner in chapter 7. Interact with the dead man in the chair to start this one. The final one is chapter 11 in the marrow restricted labs. For the final one you need to open the shutter in the morgue area. From here go down the hallway and enter the door on the right. Pick up the document on the desk. After completion of the third event you'll get the revolver and the achievement will pop.