From Halopedia, the Halo wiki
Algolis was a human colony in the Algol binary system. The planet possessed a single moon. Algolis housed a relatively small civilian population and was home to some of the Office of Naval Intelligence's research and development projects for prototype weapons, including the HRUNTING/YGGDRASIL Mark I ADS.
The Office of Naval Intelligence's Section Three had facilities on the planet, to which several SPARTAN-II augmentation chemical precursors were sent without Dr. Catherine Halsey's consent, likely in order to be researched for Project CHRYSANTHEMUM.
The colony was attacked by the Covenant at some time between 2548 and 2552, During the battle, the planet's civilians were evacuated while Marines from the UNSC Marine Corps of Engineers made a last stand against Covenant forces. The UNSC destroyer UNSC Heart of Midlothian was en route to the planet to secure the prototypical HRUNTING/YGGDRASIL powered armor from Weapons Research Facility T12A before the facility was attacked. All Covenant forces seemingly perished in the attack when a Marine sergeant known as "Ghost" detonated the powered armor's nuclear self-destruct device. The planet's survivors escaped in transport ships to a UNSC frigate in orbit that carried them to safety.
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