Sovereign

Sovereign

Reapers (Known by the Geth as the Old Machines) are a highly advanced machine race that resides in Dark Space. They wipe out life every 50,000 years.

(Spoilers Ahead)

it is revealed by Sovereign in Mass Effect. That the Reapers are the ones who created the Mass Relays and the Citadel. Its all a plan to wipe out life again and again.

Reapers will be the main enemy of Commander Shepard in Mass Effect 3.