Close Quarters Combat Weapon System

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

The multi-purpose combat knife is a simple ceramic-alloy blade mounted on a shaped-rubber grip hilt. 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.

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.