Reapers (Mass Effect)

The Reapers are an ancient race of alien synthetics that exist beyond recordable time and with motives that are beyond our comprehension. For millions of years, thousands of cycles, the Reapers have used their creations- the Mass Relays and the Citadel, to control the evolution of various races in the galaxy, allowing them to raise up to a certain point before suddenly wiping them out utterly and entirely. The last cycle was the Prothean Empire, a massive imperialistic culture that idolized warriors and fighting with a social darwnisitic viewpoint, which allowed them to hold out against the Reapers for a longer period of time and record more information for the next cycle, but in the end they were helpless as the Reapers destroyed every one of their colonies and cleaned their planets of the Prothean influence. In combat, the massive Reapers are capable of firing a "laser" of molten tungsten, superheated and fired at a fraction of the speed of light. Their most dangerous ability, however, is their ability to indoctrinate those around them for too long, turning them into mindless slaves at the worst, or even more dangerously, extremley similar to their original selves with Reaper motives, often unaware that they have even become indoctrinated until it is too late.

Many thousands of years later, Humans discovered the remnants of a Prothean base on Mars which jumped start their technology tens of thousands of years. They joined the galaxy as a major political party, alongside the Turians, Asari and Salarians, but by this point the Reaper's harvest of this cycle was already over due. The vanguard of the Reapers, Sovereign, had already attempted multiple times to bring about the actions that would allow him to bring the Reapers into the Milky Way, but was defeated each time. The last major attempt was when he secretly indoctrinated the entire Rachni species and unleashed them on the galaxy, forcing the Salarians to uplift the Krogan people to fight them off. He would eventually be discovered by Saren, a Citadel Spectre, who became indoctrinated and came close to bringing about the Reapers under the guise of a Geth invasion, but was stopped by the heroic actions of Commander Shepard, who's actions resulted in the deaths of both Saren and Sovereign. While the galaxy refused to believe the truth about the Reapers, the threat was not yet stopped, as the Reaper Harbinger was planning to bring about the arrival.

Harbinger was successful, and the Reapers easliy wiped out the Batarians on a beeline for Earth- their first major target as a personal insult to Commander Shepard, who had stopped them so many times. The Reapers could not be stopped, and Earth fell in a few hours, forcing Shepard to flee the planet and build the biggest fleet in history by uniting every group of aliens in the galaxy under a single banner to even have a hope of stopping them.

The Invasion of Cybertron
TBW.