From Halopedia, the Halo wiki
12-9F5 was a specialized M808B Scorpion tank in service with the UNSC Marine Corps. The tank featured a wider-set track and an enhanced undercarriage, making it ideal for urban warfare. As well as its modifications, the vehicle appeared to have a black chess piece in the form of a knight painted on its front left tread cover.
In 2552, 12-9F5 was assigned to the frigate UNSC In Amber Clad. In October 20 of that year, during the Battle of Earth, the tank was delivered to John-117 in Old Mombasa to assist him in fighting his way to the Prophet of Regret's carrier. The Chief, accompanied by two Marines, used the tank to cross the main bridge leading to downtown New Mombasa, destroying numerous Type-32 Ghosts, Type-26 Wraith, and Banshees along the way. However, he was soon forced to abandon the tank in a highway tunnel due to a set of blast doors preventing the tank from going any further.
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