Hyperion

Hyperion is a greatsword. It inflicts +15% damage per every additional enemy within a 65 foot radius (max +100%) of the wielder when used by Noctis or Gladio.

Source
Hyperion is given to Gladio in Chapter 14. It can later be purchased from Culless Munitions in Hammerhead for 8,000 gil. It can be sold back to shops for 4,000 gil.

Trivia

 * There are several weapons called Hyperion that appear throughout the Final Fantasy series. The foremost is a gunblade used by Seifer in VIII.
 * In Greek mythos, Hyperion is the titan of heavenly light and father of the Sun, the Moon, and the Dawn.