The energy sword is a futuristic sword made of plasma. Unlike most swords they can cut through nearly anything. Three major disadvantages are that they are heavy, they have inconsistent defense, and they have a plasma battery which needs to be recharged after so many attacks. 

Energy sword

These energy swords are favored by sangheili for close combat. Particularly, sangheili officers to show rank and spec ops sangheili.They also often serve as a secondary weapon to most ultra elites.