KaiserDragon, you are indeed correct. The answer was Penance.

"Know fear, mortals, for now you face me."
Penance is NOT a pushover by any means.

"You will pay the price for the treason you committed."
It's implied/explained that the Dark Aeons and Penance appeared because of the group defying Yevon. Most of the Dark Aeons are summoned by summoners who believe that Yuna is a traitor.

"I am no leader, I am something far worse."
"Leader" translated into German is "Führer". In the Japanese version (and some European versions too), Penance is called "Der Richter", German for "The Judge". So he's not a Führer (leader), he's something else.

"I am your judge, your jury, and your executioner."
Again refers to his Japanese name being "The Judge".

"Though you vanquished the shadows that came before me,"
You have to defeat all the Dark Aeons to face Penance.

"Against me, you stand no chance."
More bragging about how awesomely powerful he is.

"If you seek death, take the helm and approach me."
Penance is fought on the deck of the Fahrenheit. "Helm" in this case refers to the helm of a ship, Karifean.

"I will make you see the folly of your existence."
Again, more bragging.

"But before I pass my final judgement, speak my name."
Again with "judgement" referring to his Japanese name.