Cmon guys, how can you not have guessed Penance.

Know fear, mortals, for now you face me. - Basically he is the toughest mob in the game, you would do well to fear him

You will pay the price for the treason you committed. - the first Dark Aeon you faced was a fallen Summoner, who happened to call you traitors

I am no leader, I am something far worse. - Pretty simple here, he is not the leader he is the final challenge.

I am your judge, your jury, and your executioner. - Pretty simple also, He has Judged you passed down his sentence and precedes to be your executioner.

Though you vanquished the shadows that came before me, - simple, you have to vanquish all the dark aeons to be able to face him.

Against me, you stand no chance. - The weak and unprepared WILL be trashed by him, only those who cheated (with Yojimbo) and those of us dedicated enough even have a slight hope of defeating him.

If you seek death, take the helm and approach me. - You can only approach him via the Air Ship, he has his own location on the map.

I will make you see the folly of your existence. - more of the same thing from the "Against me, you stand no chance" line.

But before I pass my final judgement, speak my name. - Penance.

If I am right (which I am) I declare open round.

EDIT: Btw Dak you really need to replay the game again if you think that attacking Seymour was the only treasonous act, you don't remember killing Yunalesca? I would say (from Yevons PoV) THAT was the worst act of treason for the whole world.