Apocalypse

Apocalypse is a greatsword. When the user's HP falls to 30%, damage rate increases by +30; for every percent below 30%, damage rate increases by +9.

Source
Apocalypse can be found in the Balouve Mines maze in Chapter 15. It can be sold to shops for 10,000 gil.

Trivia

 * There are several swords named Apocalypse that appear throughout the Final Fantasy series. They are usually among the most powerful swords available.