Follow the guide I have here to get to the point where you get this achievement: Solution for Easy way out in Visage
Before you do this, you should make a save so that you can load back to it once you get the void ending.
Use all 6 bullets in the Revolver. Once you run out of bullets, go back down to the now empty Revolver and pull the trigger again. If you forgot how to get back, you need to go through the mirror down the stairs that leads to the Garage. Once you use the Revolver after it's out of bullets, you will get the void ending.