The sewer killer is the Jason Voorhees inspired murderer in lakeview cabin before you go play lakeview cabin 3.
HOW TO FIND HIM/HER: There is a reason it is called the sewer killer, if you can get the crowbar , go back to the spawn , there will be a manhole you can use the crowbar on ,once you have opened the sewer there will be hanging dead bodies , your close. Once you are there you will find NES Jason. ( aka sewer killer ) How to kill him is entirely up to you. ENDING:
When you kill the killer nothing happens , but you get a feeling of accomplishment so i guess you win. I honestly think this is just an easter egg map for the over acheivers who wanted to say , i beat the entire game. But maybe it'll come together in the end. Okay im going to stop talking now.