The Promethean arm-blade of a Promethean Knight is a melee weapon that can cause great damage to the player. The arm, being attached to the Knight, is unusable to the player. Promethean Knights can lunge at opponents to slash them with the blade and execute a powerful leap attack with it. This weapon can also throw the Promethean Pulse Grenade with great accuracy. The blade is composed of orange or blue hard light.
Different variants of Promethean Knights wield different arm-blades.
- The basic Knight wields an arm-blade with a noticeable curve near the base and a forked tip. The overall shape is similar to half of an Energy Sword.
- The Lancer's arm-blade resembles a war axe which tapers towards the base but the tip flares out at a right angle and has a crescent-shaped edge.
- Knight Commanders' arm-blades are elaborately designed with a bigger, more complex silhouette and a curved tip. The arm-blade's outline resembles an Incineration Cannon if viewed from the side. (This may be intentional as Knight Commanders wield Incineration Cannons as their primary weapons.)
- Knight Battlewagons have considerably larger arm-blades which are thickest at the tip and taper toward the base in the shape of an elongated right triangle. The base of the blade has an additional triangular tip.
- One of the assassination moves John-117 or other Spartans can perform on a Knight has him/her tear the hardlight sword off and impale the Knight with it.