Plasma charge
From Halopedia, the Halo wiki
The Plasma Charge is a Covenant air-to-ground plasma weapon utilized by Seraphs for ground bombardment. They are initially focused pillars of superheated gas, but quickly lose integrity and billow out into destructive conflagrations that can overwhelm even SPARTAN-III battle armor, as seen in the tragic Battle of Pegasi Delta, where there was sustained usage of plasma charges which were sufficient to slaughter dozens of the commandos of Beta Company in the open. The only way for the Spartan-IIIs to survive the attack were for them to find cover. These seem to be the Covenant equivalent of unguided bombs or rockets, used in a similar manner and functioning in a similar capacity.
