If you’re from the US, welcome to uncharted territory, for you, like me, have never fought the Dark Aeons before, more than likely. The Dark Aeons were bosses exclusive to the International/PAL releases of Final Fantasy X over a decade ago; luckily for us, they’re in the HD releases of the game.
So, what ARE the Dark Aeons? That, I cannot really answer, since there’s not a lot given out in the plot about them. (Or at least anything I’ll give out up front - you’ll learn what you can in due time.) What I can answer without spoiling a lot is that they’re the best of the best of the bosses in this game, sort of. They noticeably go up in difficulty from Dark Valefor (kinda like the Area Creations of the Monster Arena) to Dark Shiva-ish (the Species Creations) to the rest (Originals) to the one to end it all, Penance, who is Nemesis on steroids. Ultimately, beating Penance is the goal of this quest, and what makes it so difficult …
… but being able to achieve this is the true mark of a Final Fantasy X master. Being able to beat Penance signifies you are among the elite in FFX, for so few are able to beat a beast whose stats mark near the maximum allowable by the game. For that is why this, of all the Sidequests sections is left for last - it may not be the longest, but it is definitely the hardest.
In the following sections, I will construct analyses on how you should prepare, how to find the Dark Aeons, their mechanical data, and, finally, how to cut them in twain. Enjoy.