Close Quarters Combat Weapon System

The Combat Knife is a United Nations Space Command infantry-sized knife.

The multi-purpose combat knife has a 20 centimeter complex ceramic-non-reflective titanium carbide alloy blade with a serrated edge mounted on a shaped-rubber grip hilt and is balanced for throwing. All UNSC troops are issued with a combat knife for close quarters combat and for situations when ammo is scarce. It only appears in the novels, and on the backs of ODSTs in Halo 2.

It cannot be used in the games and might be used in Halo 3 for melee combat, possibly to duel wield and throw.

Trivia
The ODST Action Figure, made by Joyride Studios, has a Combat Knife that can be held by the figure.

Also, it was Fredric's favorite weapon. You may also notice that it appears in difficulty level badges from Normal to Legendary.

In Halo 3 Concept Art for EGM shows a Spartan with a combat knife on his right leg.