From Halopedia, the Halo wiki
Little is known of the smaller variant used by Covenant troops to breach UNSC vessels, though a larger variant has been seen with more frequency. The charge's appearance is that of a long bulbous shape with many spikes protruding from it. It has a central control hub located at the top of the bomb. It utilizes the annihilation of matter and antimatter to create an explosive.
Several antimatter charges were utilized by the Covenant during Operation: WARM BLANKET, with charges being placed within a number of CCS-class battlecruisers as decoys designed to vaporize the attacking Spartan-IIs. One of the Spartans, Solomon-069, was successfully lured onto one of the battlecruisers and subsequently killed by the antimatter charge onboard.
In another event, Cortana believed Covenant boarding parties used an antimatter charge to disable the UNSC Pillar of Autumn's Magnetic Accelerator Cannon during the opening stages of the Battle of Installation 04.
A number of these weapons were used during the early stages of the Battle for Earth to destroy Earth's orbital defense platforms. Covenant boarding parties manually delivered the ordnance and detonated them from a safe distance, destroying both Athens and Malta stations. The bomb destined for Cairo Station was secured by John-117 and Cortana. John-117 later performed a micro-gravity maneuver to deliver the device to the Covenant CAS-class assault carrier Day of Jubilation, entering a breach in the ship's hull created by a pair of Longswords. The charge was activated within the ship's reactor core, and John launched himself from the vessel shortly before its destruction.
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