Reapers

The Reapers are a highly-advanced machine race of synthetic-organic starships in the Mass Effect Universe. The Reapers reside in intergalactic (dark) space. They hibernate there, dormant for 50,000 years at a time, before returning to the galaxy.

"Reapers" was a name bestowed by the Protheans, the previous galactic power fifty thousand years before.

The Reapers are the original creators of the Citadel and the mass relay network. These massive constructs exist so that any intelligent life in the galaxy would eventually discover them and base their technology upon them – all part of a scheme to harvest the galaxy’s sentient life in a repeating cycle of purges that has continued relentlessly over countless millennia. The reason for the purges were to prevent species from advancing too far to be able to create singularities.