From Halopedia, the Halo wiki
The knight blade is a UNSC weapon attachment that is used to counter personnel in close quarters combat, but additionally can inflict heavy damage and knockback against vehicles. It is presumably based off of the same technology that the Promethean Knights use.
If a loadout weapon has this attachment equipped, it will display different melee animations and also has increased lunge range. It can destroy a Type-54 Ghost in four hits or an M12B Warthog in six hits, assuming neither vehicle is hit in its respective weak point. It is noted to increase melee effectiveness, damage, and knockback against vehicles. Unlike the lightblades used by actual Knights, the Knight Blade attachment causes enemies to disintegrate if they are killed with it.
List of appearances