Promethean arm-blade

The Blade Arm, as dubbed by fans, is a melee weapon utilised by Promethean Knights in the upcoming Halo installment, Halo 4. Promethean Knights can lunge at opponents to slash them with the blade and execute a powerful leap attack with it, as shown in the E3 gameplay demonstration. Footage from 343i suggests that Knights launch Pulse Grenades from their Blade Arms. The blade is composed of Hard light and is not available to be used as a weapon in either campaign or multiplayer.

Different variants of Promethean Knights wield different Blade Arms - the basic Knight wields a blade arm with a noticeable curve near the base and a forked tip, while the Lancer's resembles a war axe which tapers towards the base. Knight Commanders' Blade Arms are elaborately designed with a bigger, more complex silhouette with a curved tip, and Knight Battlewagons have considerably larger Blade Arms with a pointed tip.

Trivia
One of the assassination moves the Chief  can perform on a Knight  has him tear the Blade Arm off and impale the Knight with it.