Consult these maps for part 5:

Exit, go to another side room, walk until you see another door sealed with level 2 blue crystal. Equip Raizan to open it. keep walking, kill all monsters who block your path, and enter the next door. To the left of this room is a Blue Book.

• File: Blue Book

Walk to the next screen, you'll see 3 elemental orbs. Don't worry because you already have all the three. all you have to do is equip all 3 weapons, one at a time, to open the path. Once it's opened, go down and keep walking straight until you see a door blocked by spirits. You cannot enter it yet, turn around to the other way and keep walking until you see another door, enter it. You will immediately see a Magic Mirror and a treasure box.

• Item: Herb

Save your game. Continue until you see some tentacles blocking your path, cut them through and keep going. You'll encounter one Renaldo in the next section, kill him. Here, you will see 3 books on the shelves.

• File: Green Book
• File: Orange Book
• File: Byakko

Continue to the next room, kill more Renaldos, and enter the first door (double doors). Walk to the right screen (Samanosuke's left) there is a treasure box.

• Map: Keep Underground Map

Walk a little further, you will notice a robe, cut it with a basic attack to reveal another a key item. You'll have to pick it up on the the other side.

• Key Item: Rosary of Communication

Exit this room and continue to the next door. In this room, there are blue flames on both side of the walls, at the end of the room is a treasure box. Picking it up will trigger another trap.

• Key Item: Purifier Bell

Swinging blades? it's an easy peasy trap. With Purifier Bell in your possession, you can now open the door which is guarding by spirits. Open your item menu and use the bell to exorcise those spirits, then enter. In this hallway, you will see Samurai encased in glass boxes, three of them will burst out, you know what to do. After killing them, keep walking until you see a dead end, at the left corner is a Fluorite.

• Item: Fluorite

Turn around and enter the first door you see (at the L corner). Inside has 2 Renaldos, kill them and enter the last door (ignore the green sealed door for now). This is a small room with 2 items.

• File: White Book
• Item: Herb

Now go back and open the green sealed door. There is a lot more items in this room but you have to kill the samurai first. You probably notice an iron maiden cage in this room, right? yes, another Fluorite is in it. Walk further, you will see 2 files and a trick box.

• Item: Fluorite
• File: Byakko
• File: Apocalypse #1
• Key Item: Blue Key (Solution: mid, right, left)

If you have Rosary of Communication, you'll see a white energy floating above corpses. You can talk to them for more backstory. 

Backtrack to where you picked up previous Fluorite (at cul-de-sac's corner), use Blue Key to open the double doors and walk a few steps to watch a cutscene.