I recommend you do this only if you have already beaten the game...
Start a game on the hardest difficulty. Once you get the pistol, shoot EVERY person you can. When you get to the first airlock (where you go outside for the first time) kill the guard. If you've killed everyone possible up to this point, the Soul Cube should appear.
This may be a glitch- and I found it while f*cking around one day.
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