Half-Life: Opposing Force
Half-Life: Opposing Force is an expansion pack for the game Half-Life. The game was developed by Gearbox Software and Valve and published by Sierra Entertainment for Windows in 1999. Opposing Force was the first expansion for Half-Life and was announced in April 1999. Lead designer Randy Pitchford believed Gearbox was selected to develop Opposing Force because Valve, the creators of Half-Life, wanted to focus on their future games.
Opposing Force returns to the same setting as Half-Life, but instead portrays the events from the perspective of a U.S. Marine, one of the enemy characters in the original game. The player character, Adrian Shephard, is sent in to neutralize the Black Mesa Research Facility when a incident causes it to be invaded by aliens, but quickly finds that the Marines are outnumbered by enemies. The game is notable for featuring an entirely new alien faction called Race X that is never seen anywhere else in the series, as well as new weapons and mechanics like commanding squads of marines and using night-vision instead of a flashlight.