The Reaper War is a significant conflict in the Mass Effect video game series, primarily occurring in the third installment, Mass Effect 3. It involves the return of the ancient synthetic race known as the Reapers, who seek to harvest advanced civilizations across the galaxy. The war is marked by large-scale battles, alliances, and the struggle for survival against overwhelming odds. During the Reaper War, players control Commander Shepard, who must unite various species, including Humans, Asari, Turians, and Krogan, to combat the Reapers. The outcome of the war is influenced by player choices throughout the series, leading to multiple possible endings that reflect the consequences of those decisions.