Mass Relays are enormous structures, sporadically located throughout the Milky Way, that create corridors of virtually mass-free transit zones between star clusters in the Mass Effect universe. This raises the effective speed of light, allowing starships to travel at incredibly high velocities without relativistic effects. Mass relays are the most powerful mass effect engines in existence, enabling travel at speeds far greater than conventional FTL drives; distances that would take millenia to negotiate using starship-based mass effect engines can instead be traversed instantaneously via relay.
Because of their importance in travel throughout the Milky Way, mass relays typically form the centerpieces of sapient development all through the galaxy. From a given relay, settlement radiates outward via conventional FTL technology. This gives rise to star clusters, relatively small pockets of civilization scattered throughout the galaxy, separated by tremendous distances and linked to one another via relay.
Mass relays predate any of the species that make up the Galactic Council. They are thought by most of the galaxy to be of Prothean origin, constructed before their race's extinction millennia before the Council's foundation. The mass relays' technology far surpasses the engineering capabilities of any of the Council races; no new relays have been constructed in at least 50,000 years.
First Contact War
The discovery of the belligerent Rachni race prompted the Citadel Council to impose restriction on the activation of relays unless the destination on the other side was known. After Earth's discovery of the Charon Relay in the Sol system, humanity began activating every dormant mass relay they could find, unaware of the Council regulations against doing so.
Humanity's first contact with an alien race occured in 2157, when a Turian fleet discovered a Human Alliance survey team attempting to open an untouched mass relay. The Turians attacked the survey team and followed them to a human colony, which the Turians then occupied. The Alliance counterattacked, after which the Council intervened and forced a truce. The Alliance was shortly afterwards accepted onto the Citadel.
It is revealed later in the original Mass Effect that the Protheans did not actually construct all of the mass relays strewn about the galaxy. Instead the Reapers or whatever race preceded them created the mass relay network. The Reapers then used the relay network to periodically harvest all intelligent life throughout the Milky Way and thus become more powerful.
It is implied that after purging the galaxy of sapient life, the Reapers leave the mass relay network dormant to accelerate further developments of new spacefaring species for the Reapers to farm in the future.
It is discovered late in the game that the Citadel is actually a giant mass relay that connects to the Reaper resting place. Every 50,000 years the Reapers send out a beacon to the Keepers telling them to activate the mass relay, and the Reapers proceed to ambush the races currently inhabiting the Citadel, purging all life from the galaxy again.
The only mass relay ever successfully built was by the Protheans in their final years before the purge.
Found on the planet Ilos, the Conduit is the Prothean built mass relay connecting Ilos with the Citadel. In the closing years of their race, the Protheans discovered the secret to Element Zero, and were able to create their own mass effect fields. In order to prevent the Reapers from discovering it, the Conduit was built on a planet outside the Terminus Systems, not linked to by any known Mass Relays.
The Conduit is later found and reactivated by Saren Arterius. He brings an army of Geth through the Conduit and attacks the Citadel when nobody was prepared. Saren attempted to activate the Citadel himself, but was killed by Commander Shepard before the activation.
In the Arrival DLC for Mass Effect 2, Dr. Amanda Kenson discovers a Reaper artifact, Object Rho, that gives her a vision of the upcoming Reaper threat. Knowing the Reapers plan to invade via a relay she designates as the Alpha Relay, Kenson devises a project to destroy the Alpha Relay by ramming a massive asteroid into it.
After rescuing Kenson from Batarian capture, Commander Shepard discovers that the doctor has been indoctrinated, and is actually facilitating the incoming invasion. Shepard is forced to activate the dormant project, destroying Alpha Relay. Its destruction, howerver, unleashes the powerful mass effect fields controlled by the relay, creating a supernova that destroys the entire system, including the local Batarian populace.
- The current design of the Mass Relay is based on a design originally meant for the Asari warship Destiny's Ascension.