Soul Saber

Soul Saber is a sword. It grants Noctis +4% damage for every percentage point of stamina lost under 50%

Source
Soul Saber can be earned as a reward for completing the Cursed Legend quest. It can be sold back to shops for 9,000 gil.

Trivia

 * There are several weapons named some variation of "Soul Saber" that appear throughout the Final Fantasy series. Unlike the sword that appears in FFXV, these Soul Sabers often have the ability to transform damage done into MP for the wielder when the wielder lands hits on their target.