Mass Effect: Return from the Shadows
Battle for Earth
Fifteen years have passed since the Reapers, a species of artificially intelligent beings that lived on the edges of the galaxy and whose sole purpose was to exterminate all intelligent life once it reached a certain point in it evolution, were defeated. The war was costly; many worlds were reduced to rubble, millions of lives were lost, and the mass relays, the ancient devices that allow for intergalatic travel, were destroyed. It had looked like the cycle would continue, that the Reapers would succeed in their mission, eliminating the space-faring races from the face of the galaxy and leaving only ruins for the next cycle to ponder over.
However, thanks largely to the efforts of Commander Shepard, the many races of the galaxy was able to put aside their centuries old grudges and work together to combat the Reaper threat. Shepard brought the armies and fleets that he had gathered to one final battle at the homeworld of the humans, Earth. Then, using a weapon designed by the Protheans, an ancient race that the Reapers has wiped out thousands of years earlier, Shepard was able to defeat the Reapers by choosing to use the power of the Crucible to destroy all sentient synthetic lifeforms. The weapon destroyed not only the Reapers at Earth, but across the galaxy, using the mass relays to spread the destruction. Unfortunately, this blast not only destroyed the Reapers, but also any sentient synthetic life, such as the Geth and AI units, and the mass relays.