Hades is an optional boss found in Memoria on Disc 4. When you get to the ocean room where Quina tries to swim first go through that scene. When you have control of Zidane you need to search in the coral formation to the right of the screen. Note that an "!" won't appear. You just have to hit the X button until a "Mysterious Voice" starts talking to you. He will warn you that if you persist in harassing him you will die. Keep at it and you will engage in one of the toughest battles of the game.

The key to fighting Hades is to have all your party equiped to absorb shadow. Since Hades is weak to Holy, Eiko's Madeen is very effective. Who ever your strongest characters are would be the best choice for this fight.

After you defeat Hades you will gain access to the his synthesis abilities. Before trying to synthesize anything you need to first complete the Stelazzio Sidequest to obtain the Hammer. Having this item will allow you to synthesize more than one item on Hades' list. Here's what you can get: