Cocytus

Cocytus is a firearm. It inflicts ice-based damage onto targets and is thus more effective against enemies with a weakness to ice-based attacks.

Source
Cocytus can be purchased from Culless Munitions in Hammerhead, Galdin Quay, and the Prairie Outpost for 150 gil. It can be sold for 75 gil.

Trivia

 * Cocytus is a Greek word that translates literally to "lamentation." In Greek mythos, it is the name of one of the five rivers that surrounds the underworld.