The plasma sword

The Energy Sword consists of a curved hilt that, when activated, shapes and contains superheated plasma. Two small magnetic-field generators built into the handle of the weapon help form the oval shaped, ionized blades that the weapon is recognized for. The energy sword runs on battery power; an active sword, however, does not deplete the battery's energy: only when the sword makes contact with an object does the battery's energy deplete. Each strike from the sword will drain the battery by 10% of its maximum energy output, but if its user is careful, light strikes will not drain battery power. Once the battery power dies out the sword will deactivate and be rendered useless unless recharged. It is unknown how the battery is recharged. The Energy Sword also contains a fail-safe device that can be activated if the Sangheili wielding it drops it; the fail-safe option is always on, and a Sangheili must deactivate the fail-safe by applying pressure to the hilt before he can safely store the weapon. When the sword is dropped without disengaging the fail-safe, the weapon will deactivate the magnetic field without first dispersing the plasma, consuming the handle and thus destroying the weapon. The failsafe is included to prevent enemy infantry from acquiring the weapon and using it, and is yet another insight to Sangheili tactics.I have got this info from halopedia.com