Converging beam cannon
From Halopedia, the Halo wiki
Converging beam cannons are a kind of weapon system employed aboard Forerunner warships.
Converging beam cannons are directed-energy weapons that fire lances of exotic energy upon enemy vessels and provide a path for follow-on antimatter streams, which destroy everything they touch. The energy fires can be curved toward targets using delicate strands of invisible force and tracing pathways, usually wielded by a Warrior-Servant operator. The weapon's energy can be varied, though more powerful beams have a less smooth motion as even Forerunner science was unable to contain the destructive energy of the weapon.
Twenty-eight of these weapons are part of the armament of the Sojourner-class dreadnought, while the Mantle's Approach employed 25,000 these such weapons. The Guardian Custode each carry six converging beam cannons, which were considered more than adequete to defeat fleets of primitive capital ships, but did little when facing Forerunner ships controlled by the Flood.