You may be thinking where you get Vincent's level 4 Limit Break and final weapon, well it is here in the waterfall although if your careless you may miss it completely by disc 3.

This must be completed on disc 2!

REQUIREMENTS

Must have Vincent and he must be in your party when entering the waterfall. You need either the submarine or a chocobo capable of crossing mountains.

WHAT TO DO

Get in your submarine and ride it to the docking area near the Gold Saucer desert. Dive under water and there should be an underwater passage right in front of you. When you pass through and surface you'll be in a small area of water beside a waterfall. Dock on the side and walk into the waterfall. Make sure you have Vincent in your party. When you enter, watch the scene (disc 2 only).

Once the scene is over leave the area and continue with the game, main story or leveling or anything. After a certain amount of time has passed (couple hours is a good estimate, though I'm not exactly sure what the requirements are), return to the waterfall with Vincent and examine the glowing pedestal to receive Death Penalty and Chaos, his final weapon and level 4 Limit Break.