Head to the Abandoned Dungeon entrance and as you enter, head to the cells to the left to find and speak to Doujun. He will hand over the Surgeon’s Bloody Letter and ask you to send him someone.
You can lure either Jinzaemon or Kotaro however note that whoever you send, you’ll be able to complete their quest on this playthrough. Doujun will thank you when you return and hand over three lumps of fat wax. Rest at the nearby Idol and Eavesdrop on Doujun before resting a second time and speaking to him again, this time obtaining the Surgeon’s Stained Letter.
After Mibu Village
Once you head through Mibu Village and defeat the Corrupted Monk at the end, you’ll obtain the Mibu Breathing Technique. Return to the large lake in the village and dive under to find a Carp with red eyes who, upon defeated, will relenquish the Red Carp Eyes. Return to Doujun and this time he will hand over two lumps of grave wax.
Returning to the Abandoned Dungeon
Return to the nearby Idol and rest up before making your way down the path to reach the water. Head to the small piece of land to the left to find a hostile Jinzaemon or Kotaro with red eyes, defeat them and return to the Idol to rest again.
This time, once you reach the water, swim under and head left to find a path that leads to Doujun’s cell. Speak to Doujun and pocket the nearby Dosaku’s Note before opening the cell and returning to the Idol to rest again.
Return to where you defeated Jinzaemon or Kotaro and this time, a hostile Doujun will be there with red eyes, defeat him to obtain the Academics’ Red Lump and conclude his questline.