Reapers

The Reapers are a mysterious race of powerful, killer robots. Their origins are unknown, as they claim to be part of a circle, with no beginning and no end. Their purpose is clear, however. They maintain the growth of organic life, which they see as highly inferior due to their life spans. They oversee societies as they grow, discover the Citadel and Mass Relays, and then become powerful. At the apex of the society, the Reapers strike, eliminating every person and every trace of the society, leaving the Citadel and Mass Relays for another race to discover.