Shepard and his squad follow Saren to ancient Prothean ruins on the planet Ilos. Here, they are greeted by a computer AI named Vigil who begins to explain everything the Protheans had learned about the Reapers. Of all the troubling information Vigil reviews, the biggest shocker comes when he explains the Citadel's purpose. The Citadel is actually a giant Mass Relay, the mass transit devices ships use to traverse the galaxy. This Mass Relay, however, allows the entire Reaper army to come through from Dark Space, the area just outside the Milky Way where the Reapers wait.