In your travels, you might have come across a waterfall that will throw you back if you try to go through it. That said, you might have come across a steel door at the bottom of the Chapel, that you cannot get through. Both of these lead to a section known as the "Forbidden Area". This is not a need to complete area to finish the game, but you will find a great sword and armor inside here.
I know what you are thinking...Well, I can't get through either way, no matter how hard I try. Do not worry, Bahamut is here to guide you through this. And trust me, the trip is so worth it.
First off, we need to go soul searching. You should already be in possession of two major ones. The opposing Undine and Skula Souls. One allows you to walk on water, while the other allows you to move underwater. These are easy, as they are gotten through the progression of the story. The second soul you will need can be one of two different souls. Either you will need the Manticore, or the Devil Soul. Either is fine, but it is the key to getting through the waterfall.
Now that you have your souls, head down to the waterfall. If you are uncertain of the room, it is the third room to the right of the bottom most warp point. When you are in the room, stand in the left most part, before you hit the water. Equip the Undine and Manticore/Devil souls. Now transform into your choice of beast, and head to the right. If done correctly, you will go through the waterfall, before you transform back into Soma.
Right in front of you sits the Eversing Mail. This is one of the best armors in the entire game. Get her on fast, and continue over to the right.
Continue to the right, fighting all the enemies, until you come to a place you can go up. Kill off the two gargoyles, and scale your way to the top. In the next area, you will see that all too familiar steel door, with the lever to the left. Hit the lever, and it will make the door dissapear. This will lead you back to the Chapel area. Rather than go there, head back down, and continue to the right at the bottom.
In the next area, you will enter a large room with not only two Harpies, but a very unusual pirate ship. (Goonies anyone??) You must first get on the ship, before you can stroll any further. In the center, you will see steps that go left and right in the water. Now is the time to Equip Skula. Head over to the right, and go all the way to the end of the areas, to receive the Joyeuse sword. After you looked at it, i am sure you are a bit upset at the fact that in retrospect the sword is not a awesome sword like I said before. Before you go emailing me with letters of wasting your time, there is a small place you have not checked yet. Look on your map. You should see a small room in the bottom left corner of the ship room that has not been explored. Head back to the ship, and go to the left down the stairs. Fight all the Killer Fish, and the pillars. Dead end...or is it.Strike the wall a few times, and it will open. enter the room, and you will see the real reward right in front of you. It is the Claimh Solais. This is one of the best weapons in the entire game. It has a +103 on attack, as well as stat boosts for every other stat. Only the Excalibur and Death's Sickle have higher attack stats.
With this and the Eversing, you should have no trouble getting through the rest of the game. Well, what are you waiting for, get out there, and try out your new toys!