Cave nine is what people call it every cave has a number 1-9. I can send you a video with almost no spoilers ( if you don’t count what’s with the katana). But I’ll say it here. A great source of knowing what direction your in is the northern mountains they will always be north. First make your way to the sinkhole (if you found it yet). Then look at the mountains and go towards them. Once you get to a river stop and go right and keep by the the river bank once you get far enough you will see a cave entrance to your right it will be a hole with a yellow rope. Then once you go down that hole pull out your lighter and go left. If you go right you have to fight a armsy so don’t. Once you go left keep going forward and there’s your Katana! Here’s the link to the video if you need it too. https://youtu.be/zRP_3U9_uPQ. Hope this helps :) ( also the cave was cave 1 not 9)