Reapers

The Reapers were an ancient race of robotic Droids tha existed in the galaxy of Galactic Prime for hundreds of thousands of years. Their creators presumably were an unknown, now-extinct organic race.

Every 50,000 Years
Once every 50,000 standard years, the Reapers waged war on sentient organic life across the galaxy, trying to exterminate the organics.

Capabilities
The Reapers traditionally had millions of robotic soldiers and hyperdrive-capable capital ships and starfighters.

War with the Galactic Republic, Jedi, and Mandalorians
In the years 2333 to 2331 BBY, the Galactic Republic and the Jedi Order teamed up with Mandalorian warrrior clans and mercenaries to defend the so-called Known Galaxy against a Reaper horde attempting to wage genocide against sentient organic life.