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 (8 inches) blade made of high carbon steel coated for corrosion resistence and is 43.3 centimeters (1 foot, 5 inches) sheathed. All UNSC troops are issued with a combat knife for close quarters combat, for situations when ammo is scarce and is also and is balanced for throwing. 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.

ODST's
The only time a combat knife is seen is on the backs of Orbital Drop Shock Troopers, though they never use it.

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, a Spartan is shown with a combat knife on his right leg.