Half-Life: Opposing Force is a 1999 expansion pack for Valve’s seminal first-person shooter Half-Life, developed by Gearbox Software. It reimagines the original Black Mesa incident from a fresh perspective, placing the player in the boots of Adrian Shephard, a U.S. Marine sent to contain the catastrophe. Opposing Force expands Half-Life’s narrative, world-building, and gameplay with new enemies, allied NPCs, weapons, and a distinct tone that complements the original game while standing on its own.