From Halopedia, the Halo wiki
The Lightblade, also known as hardlight blade, is a Forerunner melee weapon composed entirely of hard light. It is affixed to the arms of several Forerunner combat constructs, most prominently the Promethean Knights.
The blade performs similar to an Energy Sword in that it can strip away energy shields and cut through armor with relative ease. The blade is unwieldable to players as the weapon disintegrates along with the rest of the body immediately after the Knights are destroyed. However, it is usable by players when they are assassinating a Knight, whereupon the player tears the blade-arm off the Promethean and uses it to stab its former owner in the chest for the kill.
There are 9 types of Hardlight Blades in the games, a unique type per Promethean Knight class (Basic, Lancer, Battlewagon, Commander, Marshal, Strategos, and Champion). The eighth type, used by the Warden Eternal, functions differently from the others, as it starts as a closed up disc shape, and then hardlight extends from two ends. The ninth type, known as the Knight blade, is used by SPARTAN-IVs as a weapon attachment for the following loadout weapons: MA5D assault rifle, BR85N battle rifle, and M395B DMR.
List of appearances