The basement (地下室 chikashitsu ) is an area located inside the mansion.
Two Places Edit
In Ao Oni, there are two areas of the mansion called the "basement." The first of these has green floors and many bookcases. Before you enter this room, just after obtaining the basement key, Mika's scream is heard. You must go check on her before you can advance. You find that Mika has been killed. After picking up the basement jail cell key and proceeding north, the player will find Mika. She will then turn into an Oni and begin to chase the player.
When you enter the basement, the first thing you may notice is a shiny object placed on top of the nearest bookshelf to the entrance door. To obtain this item, go west to the new room (しん部屋 shin heya ) and there will be a chair inside. Before you move the chair outside this room, move the chair toward the object placed on top of the central bookshelf in this room to obtain a flat head. Then push the chair toward the initial bookshelf to obtain the basement jail cell key.
When you head north, you will encounter Mika who transforms into another Oni, and chases you just as the other one. (This means that there are now two Onis wandering the mansion.) You will need to escape just as the Onis before her. Once the coast is clear, go back to the basement. Go east to enter another room, then continue east and you will be in a completely dark room known as the basement jail (地下室 chikarō , lit. "dungeon"). Use the lighter and find your way to the jail cell's door and use the basement jail cell key. Once inside the cell, light the candle. Go back outside of the cell and view the writing on the walls from behind the bars. If done correctly, there should be four numbers visible.
Go back to the main room of the basement and move the two bookcases on the northern wall, revealing a panel held tight by four slotted screws. Use the flat head to replace the head of the driver, and the Phillips head screwdriver will change into the flat head screwdriver. Use the flat head screwdriver to unscrew the panel to reveal a vault beneath. Enter the code from the basement jail cell to obtain the annex key. From there, head east and upon attempting to open the door, an Oni will appear from the closet. After escaping from this Oni, you can return to this room and open the door, leading to the stairwell (階段 kaidan ). From there, you can unlock the north door with the annex key to gain access to the annex.