Javelin

Javelin is a polearm. When the user's HP falls to 30%, critical rate increases by +5; for every percent below 30%, critical rate increases by +1.5.

Source
A Javelin is equipped by default on Noctis. It can also be purchased from Culless Munitions at Galdin Quay and the Prairie Outpost for 150 gil. It can be sold back to shops for 75 gil.