From Lexicanum

Measuring up to 20 inches of gleaming steel, the Catachan Fang is the most common type of the knife used by the Catachan regiments. Equally useful for cutting your way through jungle or deviant Eldar, the Catachan Fang is renowned throughout thousands of star systems. This knife is also used for settling disputes between Catachans. Usually, one knife is placed in the centre of a fighting pit and the combatants must each strive to get hold of the knife. the duel ends when one fighter draws blood, though this can often mean death as well.

Sources

• Codex: Catachans (3rd Edition)