Catalyst

The Catalyst is an ancient and powerful entity of unknown origin that resides within the Citadel. Its nature is unclear; it is identified as a machine, but it is never specified whether it is an AI, VI or something else entirely. Before being encountered by Commander Shepard, the Catalyst was believed to be the final component necessary to complete the Crucible.

Purpose
The Catalyst serves as the architect and overseer of the Reapers and their cycle of destruction. As it explained to Commander Shepard, the Catalyst was tasked with solving a dire problem: the inevitable creation of synthetic intelligence by advanced organic civilizations, and the equally inevitable conflict that results. It chose to resolve this problem by putting in place a system which would prevent any advanced civilization from reaching that point.

The Catalyst's solution took the form of the Reapers. Space-faring organic species would be harvested and processed, with millions of bodies and minds from each race being processed and converted into new Reapers, even as the Reapers themselves worked to destroy their civilizations. By doing this, the Reapers preserved the harvested races, while allowing more primitive races to advance and ensuring that the threat of complete annihilation of organics by synthetics was averted.

Discovery
When encountered by Commander Shepard, the Catalyst represented itself as a transparent projection, taking the form of a young boy that Shepard had watched die during their escape from Earth. The Catalyst explains that Shepard's actions have caused it to rethink the cycle; it cannot, however, bring about a change without Shepard's intervention. See Priority: Earth for further information.

Trivia

 * It has been suggested that the Catalyst may be related to the "beings of light" sought by Kumun Shol on Klencory.