Algolis Invasion

The Battle of Algolis  was a battle that took place in the Algol system during the Human-Covenant War, likely during or after 2548.

Ground battle
Between 2548 and 2552 the UNSC colony Algolis was attacked by the Covenant. Their main objective was the destruction of Weapons Research Facility T12A, where the UNSC was developing highly-advanced weaponry for future use against the Covenant. While the civilians were evacuated by Marines, a team of engineers were sent to a UNSC testing facility to enact the Cole Protocol, destroying navigation data and a prototypical combat exoskeleton. The Covenant attacked the Marines in full force, decimating their defensive line and pinning down the survivors. In the unfolding chaos they destroyed several of the aircraft ferrying civilian colonists off the planet, killing hundreds in the process. However, one Marine combat engineer, "Ghost", went against orders and equipped the armor slated for destruction, hoping to redeem himself as a promise to a now dead comrade.

Ghost managed to destroy a large number of Covenant forces, allowing the Marines and civilians to escape. Using the suit's superior armament, he smashed the Covenant assault and laid waste to most of their ground vehicles and aircraft. When damaged and cornered, Ghost gave a code word activation, "Be Human", and activated a nuclear self-destruct in the armor, killing himself, and all Covenant forces in the area. The weapons facility was consumed by the blast, depriving the Covenant of any classified data they had sought to obtain. Marines fell back to a UNSC Frigate in orbit over the planet along with the surviving civilians, and moved to rendezvous with the destroyer.

Space battle
However, Covenant special operations forces boarded The Heart of Midlothian as soon as it dropped out of slipspace and killed the crew in their entirety save one Orbital Drop Shock Trooper, Michael Baird, and disconnected the shipboard AI, Mo Ye. Baird was in the surgery unit and was informed by the ship's AI, Mo Ye, that she was unable to self-destruct the ship as per the Cole Protocol due to a restriction placed on her in her disconnected state which did not allow her to willingly harm a human.

Mo Ye guided Baird to the bridge of Midlothian whereupon an Special Operations Elite was "tricked" into killing Baird so as to allow Mo Ye to destroy the ship. The Heart of Midlothian and the Covenant battlegroup surrounding it were destroyed.

Aftermath
Little is known of the outcome of the battle. While the Covenant were defeated, the humans suffered substantial losses and were presumably forced to abandon Algolis altogether. However, the Covenant never acquired the technical data behind the combat exoskeleton used in the battle and, even more importantly the Covenant were unsuccessful in acquiring Earth's location from the databanks on the Midlothian.