Catalyst

The Catalyst is an ancient artificial intelligence that resides within the Citadel. It embodies the collective consciousness and memories of the Reapers. 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 created by an unknown race who believed that organics and synthetics would inevitably conflict with one another. To prevent such events from happening, they created the Catalyst to oversee relations between organics and synthetics. However, as the two conflicted frequently, 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 final solution took the form of the Reapers. The physical design was created by the same unknown race, while the Catalyst was able to control them. This gave the Catalyst the ability to implement its solution by processing the unknown race into the first true Reaper (An act the race disagreed with.) The solution dictated that all space-faring organic and synthetic species would then be harvested, 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. The Catalyst explains that Shepard is the first to stand at that chamber to converse with it, which caused it to rethink the cycle; it cannot, however, bring about a change without Shepard's intervention. See Priority: Earth for further information.