Ninja Gaiden II THE UNDERWORLD ABYSS
Grab the item off of the recently deceased Volf.
Get whatever you need from Murumasa’s shop, then go through the doorway. Once you get to the end of the corridor of sorts, there will be a somewhat open area and a huge skull in the middle( because you got all 30 skulls).
Get the giant skull then go to room to the right of it. Save, then go back out to the room where you got the giant skull. This time go to the room on the other side. You now have these weird cocoon like things that block your path.
After you hit them a couple times, they will break and either some ninjas or one of those huge spider guys will jump out. My strategy for getting around these is equipping the scythe, then hitting the cocoon thing twice. Then I use an "Extermination Sweep" on it. It doesn't always necessarily kill whatever is inside the cocoon thing, but it does deal a good amount of damage.
Once you get to the end of the cocoon area, save the game and go down the path for a little ways to come to a large area with a huge drop off.
Get the few chests and the dead guy in the area if you want. Then jump down the drop off to start the chapter 13 boss fight.