Events that occurred in the year 3186 CE.
- December 15: The Third Battle of Earth ends. The joint Citadel-Federation fleet, supported by the Quarian Flotilla, the Cardassian Union, and the First Federation, assaults the Citadel, now under control of the Reapers. The fighting begins in space, in orbit around Earth, where the allied fleets engaged in a massive frontal attack on the Reapers stationed to protect the Citadel. Commander John Shepard descends to the surface of the planet with a strike team to attack a conduit in the ruins of London. Shepard ascends into the Citadel itself, where he is confronted by the Catalyst, an ancient AI created by the Precursors, who, unbeknownst to him, is a puppet of the Flood. The Catalyst attempts to use the Logic Plague to sway Shepard to its side and activate the Crucible, which will allow the Flood to invade the galaxy.