Yup. It's the Ultima Weapon. It gets stronger the more jobs you have mastered and the lower your health is. In the hands of a master Onion Knight on the brink of death, it's probably among the most powerful swords in existence.

[Assuming that knowledge is free game since it was provided to us by the mods in official parentheses and whatnot. Plus, Chiaki's a gaming nerd, so she can probably guess well enough.]