Walk all the way to the end during The Sorrow fight to unlock the Spirit Camouflage. In the dream sequence where you face The Sorrow (at the stream with lightning and ghosts), walk all the way forward until you see The Sorrow's ghost hovering over his skeleton. Let him kill you while standing over his skeleton then press R2 when the screen reads Game Over - Time Paradox to access the Revival Pill. If done correctly, you should now have The Sorrow's Camouflage in your uniforms. The camouflage's special abilities eliminate all footstep noises from you. You can also drain enemy stamina in CQC holds and add it to your own.