Hades is an optional boss in the final dungeon. He's rather difficult to find, so it's not like you'll accidentally run into him most likely. He's a challenging optional boss if everyone is not over LV50.
Preparing for this is important. Waltzing into this unprepared is a nice way to game over pretty fast. Hades has Curse, the games craziest status ailment move. It throws tons of status ailments on you. So unless you're a really high level, you're going to have to unequip a lot of amazing abilities like Auto-Regen, and maybe even Auto-Haste. Abilities like Antibody and Clear Headed should get priority if you can't equip most status ailment guards. I'm not sure what Curse is fully capable of, so beware of Stop as well. Loud Mouth on your magic users and Bright Eyes on your physical should be set up as well.
Body Temp ability on everyone is a must because of his Freeze/Mustard Bomb move. When that's all set up, now you can fight him.
He will generally use the following moves: Reflect, Freeze, Mustard Bomb, Cleave, Judgement Sword, and his charged attack will either be Curse or Doomsday. Having Body Temp nullifies Freeze & Mustard Bomb. Judgement Sword reduces one char's HP to 1, nothing too scary. Reflect means you just have to be careful not to spam magic moves on him if his turn will come before yours.
Cleave is a physical attack that hits everyone. He tends to use it after Curse (and probably after Doomsday as well). With Curse it's scary, because everyone will be in Tiny status, so you'll take more damage. So Curaga after Curse as soon as you can and spam Remedy and hope you don't get OHKOed by it if everyone is in Tiny status.
As for Doomsday, that move hits everyone. It's a rather powerful one, but can be guarded a little by Dark resistance armor/accessories.
He takes so long to charge you can fairly easily take him out before he even does a charged move if you walk into this fight with everyone who can do 9999 moves. The following are best: Steiner's Shock, Freya's maxed out Dragon's Crest, Eiko using Holy (Hades is weak to Holy), and I believe Reflect x2 with Vivi makes Flare do 9999. If you are crazy enough to max out Zidane's Thievery move, then that'll do 9999 too. If everyone is spamming 9999 moves from the beginning you'll end him rather quickly, as he only has 55k HP.