First, follow the path to reach the Dungeon Memorial Mob and Abandoned Dungeon Entrance sculptor’s idol. Go past that and continue downstairs to enter the Abandoned Dungeon. There are a lot of undead/diseased enemies here that don’t die so don’t bother fighting them. However, there are a lot of items to be found in this cavern so explore as needed. Large black insects gather near the item pouches so make sure to take them out before they can damage you.
Doujun the Surgeon
This NPC will give you the Surgeon’s Bloody Letter as part of a sidequest, requiring one “tough man” to be used as an experimental subject. This is one of the (bad) options where you can send the gentle giant Kotaro in Senpou Temple.
Once you’re done exploring the area, continue following the cavern until you reach the Underground Waterway sculptor’s idol beside a cage. Continue down the path and get the Scrap Iron along the way.
There’s a waterway that you have to swim to but you have to swim fast since there’s a large predatory fish that will attack you if you stay too long in the water. Swim across the water until you reach the dock along the way, with three enemies. Take them out and loot the items.
Jump in the water again and swim fast to the next dry land. There’s a musketeer and a swordsman guarding the path so take them out quickly.
Continue to the next room and operate the mechanism to call in the lift. Hop on it then pull the handle again to lift yourself up. Continue upstairs to find the next sculptor’s idol. This is Senpou Temple’s starting area.