The Battle of Algolis was a battle that took place in the Algol System at an unknown point in the Human-Covenant war.
At an unknown time during the war, the UEG colony Algolis was attacked by the Covenant. While 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 prototype suit of power armor. However, one Marine engineer, "Ghost", went against orders and equipped the armor, 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. 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. Marines fell back to a UNSC Frigate in orbit over the planet, and moved to rendezvous with the UNSC The Heart of Midlothian.
However, Covenant forces boarded the UNSC Destroyer Heart of Midlothian as soon as it dropped out of Slipspace and killed the crew in their entirety save one ODST, Michael Baird, and disconnected the shipboard AI. 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 Elite was "tricked" into killing Baird so as to allow Mo Ye to destroy the ship. As a result of Midlothian's self-destruction the Covenant battle group that surrounded it was completely destroyed.