Traverse Town 3

You'll find yourself at the Vacant House in Traverse Town. After the cut-scene, go talk to Cid. He'll tell you to find the other Navi-Gummi, so head for the Secret Waterway at the 2nd District. Here, talk to Kairi and examine the mural on the wall to get the other Navi-Gummi. Go back to Kairi for a long cut-scene. You'll receive the Oathkeeper keyblade afterwards.

Head back to the 1st District and talk to Cid again. He'll install the Navigation Gummis for you. Also, don't forget to take the Fireglow gem to the Fairy Godmother for the Mushu summon.

Now you must go back to Hallow Bastion using the new warp hole. When you land at Hallow Bastion there will be a cut-scene. Go to the entrance of the Castle and activate the white trinity to get a Firaga-G. Make sure your party includes Beast. Go back to the Grand Hall. First go up the stairs and there will be a cut-scene. Walk up to the keyhole to be taken to the Dark Depths.

Boss Battle - Behemoth

This battle is pretty easy. Just jump on his back, lock on the head and attack. Once his horn begins to gather energy, that means he's about to unleash a somewhat devastating lightning attack, so get away from him to avoid it. Cast Aero on yourself and heal when necessary... that's about it.

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After the battle, there'll be a short dialogue between Sora and the princesses. You'll also get the Firaga upgrade. Now all that's left is the End of the World, so feel free to finish whatever side quest that needs completing. Also, don't forget to go back to the Library in Hollow Bastion and talk to Aerith for the Ansem Reports 2, 4, 6, 10, then to Belle for the Divine Rose keyblade. Be sure to go back to the Grand Hall as well for the Oblivion key chain and Puppies 61-63. When you're ready, ride your Gummi Ship to the final area.