Zwill Crossblade

Zwill Crossblade is a dagger. It grants the wielder a +80% to damage when the wielder is at full health.

Source
Zwill Crossblade can be earned as a reward for completing the Wondrous Weapon quest. It can be sold to shops for 10,000 gil.

Trivia

 * There are many daggers throughout the Final Fantasy series that are called some variation of 'zwill.' They are usually among the most powerful daggers available.