Imagine, if you will, an island off the coast eternally beset by great and terrible storms. At the center of the storm, between flashes of lightning, you can see a spire reaching to the heavens.
How long before some brave soul tries to reach this island?
That is the great mystery! What race that has reached sentience and developed culture, art and religion could possibly ignore the mystery of the lightning-wracked spire? It sits there, taunting them. It is a challenge waiting to be overcome, an insult to the lack of ingenuity and the low intelligence of the race growing up in its glow. How long before some brave soul tries to reach this island and proclaim himself master over it?
This is a mass relay in Mage Effect. It is an island eternally wracked by lightning, wind, and rain. The relays each sit just a few miles offshore waiting for brave and foolhardy souls to venture near. Once a ship pierces the howling maelstrom, it is enveloped by the storm and disappears. To all observers it would appear that the ship has been destroyed, all but erased from memory.
In reality, the ship has been hurled through space to some other point on the shell seemingly instantly. It will sail on unharmed in new waters. As the crew looks about they find that they are sailing away from a storm very much like the one they sailed into but they are not where they had been just moments ago. Land is always near and the crew can explore this strange, uncharted land at their leisure. Often they will encounter some new race they have never met before. This new race is usually more versed in the matters of relay travel than the newcomers and make peaceful overtures. Thus an exploring race is welcomed to the Citadel’s influence.
Each relay is capable of connecting up to eight other relays, though it is rare for one to connect to more than two or three. A ship’s heading (north, northeast, east, southeast, south, southwest, west, northwest) determines which relay it connects to. The notable exception is the chain of dozens of relay islands surrounding the Citadel. These relays all connect to 8 other relays, making the Citadel the heart of the relay network.
The asari were the first of the living races to master the intricacies of relay travel. They discovered the Citadel, a palace-island of remarkable beauty. When they encountered other races, the asari offered up the Citadel as a neutral place for all races. It is a place for peace where all who are willing to treat fairly are welcome to stay. Here they founded the Citadel council, a governing body for all races that recognize the Citadel’s authority. In that time, dozens of races have come to the Citadel and been counted amongst its ranks.
Common wisdom holds that the protheans, that ancient and extinct race of surpassing magic and artifice, built the relays and the Citadel. Common wisdom is wrong. The relays, like the rest of the shell, were built by the Reapers. By design, the relays connect various points on the shell and are the fastest, most reliable method of travel. The shell is so large that without the relays none of the races would ever encounter one another. As races mature and discover the relay system, they come to rely upon it for trade and travel. This dependence is used by the Reapers when they return to wipe out all sentient life in the shell.
The Reapers first disable all of the relays, stranding the disparate races where they are. They then spend the next several centuries eliminating the various civilizations and colonies. While the relays are disabled for the lesser races, the Reapers are able to activate them at will. This cycle has repeated itself hundreds of times over.