Energy sword
From Halopedia, the Halo wiki
The Energy Sword is carried by high-ranking Elites. They have similiar ceremonial functions as swords in today's modern armies. While not often used in combat they do provide a good last ditch weapon.
The blade itself seems to generate plasma energy in a similiar way as the Covenant plasma weapons. In a plasma gun the energy is shot out like a bullet, but the energy from the sword is held into a blade-like form with small magnetic field generators built into the handle mechanism.
In Halo if the wielder drops the weapon a fail-safe activates and deactivates the magnetic field without first dispensing the plasma consuming the handle thus destroying the weapon.
In Halo 2 many high ranking Elites will use a Energy Sword when engaged in close quarters combat. For the first time, Master Chief is able to wield a Energy Sword.
Several times when playing as the Arbiter you begin the mission with a Energy Sword.
