Siege Of The Citadel

The Siege Of The Citadel was a massive battle between an army of heretic Geth led by Sovereign and Saren against the Citadel and later the Fifth Fleet of the Human Alliance. While Soveriegn led an army of Geth ships against the Citadel Fleet, Saren infiltrated the Citadel using the Conduit in order to sabotage the station from within and let Sovereign activate the Citadel's mass relay to bring the rest of the Reapers back from dark space. Organic life in the Milky Way would have been exterminated once more if not for the intervention of Commander Shepard, his squad, and the Fifth Fleet of the Human Alliance.

The battle had enourmous consequences for the galaxy. The geth were defeated and Sovereign and Saren were killed, halting the Reaper Cycle for the first time in recorded history. Humans also gained an expanded role in the galactic stage and became a major player in the Council.