Amatsu Dungeon Entrance Quest
1. Speak to the Guard Soldiers in Amatsu (amatsu 164, 174 and 171, 164). He will tell you not to enter the house. In there is the master's mother and she has been sick for several months.
2. Now, go to the castle (amatsu 85, 235) and find the Lord of Palace (ama_in02 200, 176) on the 4th floor. Tell him you heard his mother is sick. He will ask you to cure his mother.
3. After this, head back outside and go to a boy named Kouji near the walls (amatsu 189, 165). Speak to him twice.
4. Head north and go to the shrine (ama_fild01 174, 331).
5. Talk to the NPC named Kitsune Mask (ama_in01 179, 173). He'll teach you how to get rid of the ghost.
6. Go into the house in step 1. Speak to her until the ghost leaves. If you fail to get rid of the ghost each time you talk to her then she will kick you out; just keep trying. When you succeed, you will see dead Nine Tails (you have to have them in your inventory in order to complete this quest).
7. Talk to the Lord of Palace again. He'll thank you for your work, and give you ato the dungeon. The dungeon is located on the 5th floor. Simply walk up the ladder you see as you enter the 4th floor, and speak to the NPC waiting there, and he'll let you in.