OK, it looks like you got your answer, but I just sent you a PM back with what to do.
In fact, you'll face all the bosses you fought before, of sorts. Take a pic with each one. Then, just get to the end of the 'River Styx.' When you touch the corpse of The Sorrow, you die. Take the revival pill, and you'll wake up with the Sorrow's camo in with your other suits. With it on, footstep noise is eliminated. It's an awesome thing to have when you play on the more difficult levels.
But, when you play the game again, make sure you kill Raidenovich to see him walking naked in the River Styx. Take a pic of him to have a funny pic. But he comes really quickly when "fighting" The Sorrow again. But, if you could care less, and since you have his camo already, just kill yourself, then take the revival pill to skip the whole battle.