Half-Life Opposing Force is a first-person shooter video game developed by Gearbox Software and released in 1999 as an expansion to the original Half-Life. Players assume the role of Gordon Freeman's rival, Adrian Shephard, a soldier sent to contain the alien outbreak at the Black Mesa Research Facility. The game features new weapons, enemies, and gameplay mechanics, enhancing the original experience. In Opposing Force, players navigate through various environments while battling both alien creatures and military personnel. The storyline expands the Half-Life universe, providing a different perspective on the events occurring during the original game. The expansion received positive reviews for its engaging gameplay and innovative features.