Protheans

The Protheans are an ancient alien race in the Mass Effect Universe which vanished over 50,000 years ago. The Protheans arose from a single planet and developed an immense galaxy-wide empire encompassing many other spacefaring species.

The Protheans have been credited with creating the Citadel and the mass relays, feats of engineering that have never been equaled and whose core mass effect field technology forms the basis of contemporary civilization.

They are at least 70,000 years old. After achieving spaceflight, the Protheans discovered the ruins of a previous spacefaring race, the Inusannon, and from those ruins learned about mass effect physics and developed FTL technology.

Eventually they entered a devastating war with the Reapers. After centuries, the Reapers had killed or enslaved every Prothean in the galaxy, and stripped their worlds of resources. The Reapers then retreated through the Citadel into dark space and sealed it behind them.