From Halopedia, the Halo wiki
A plasma lance is a kind of energy projector employed by Covenant starships as an ultra-heavy anti-ship weapon. Plasma lances fire tightly-focused streams of plasma at their enemies, dealing a great deal of kinetic and thermal damage. At close range, plasma lances are capable of cutting un-shielded warships in half.
Scaled-down forms of plasma lances known as plasma emitters may also be present on larger warships, intended for use as general-purpose anti-ship weaponry. Plasma emitters emit continous bveams of plasma capable of being used to bore into a planetary surface to excavate Forerunner artifacts.
Unique and individual designs
Heavy and superheavy plasma lances
Light and regular plasma lances