Okay so first of all you need to get to the end of Stage 5. You will see the portal to the last boss as in the picture.


Then instead of going through the portal you go back a little and walk by the side of the cube.


You will have walk a bit and then you will see another portal.


Finally go through the new portal and get teleported into a room with some weapons, supplies and corpses. There will be three different logs which wll tell you THE TRUTH.

I hope this guide helped, bye!