Flayer

Flayer is a polearm. When wielded by Noctis or Ignis, finishers using this weapon will deal +80% damage.

Source
Flayer can be obtained as a collectable in the Steyliff Grove maze in Chapter 15. It can be sold to shops for 10,000 gil.

Trivia

 * The Flayer's Japanese name translates literally to Fang Tian Hua Ji (which has been translated into English as "Sky Piercer"), a reference to the ji (polearm) supposedly used by the 1st century Chinese warlord Lü Bu as depicted in the famous historical novel Romance of the Three Kingdoms. The ji was commonly used in Ancient China, and it was a combination of the spear and dagger-axe.