Catalyst

The Catalyst is believed to be the last part needed to complete the Crucible. Its function is unknown.

The Catalyst is an ancient and powerful entity of unknown origin. Its nature is unclear, there being no reference to it being organic, synthetic, or any other alternative. It resides within the Citadel.

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 all other organics by synthetics was averted.

When a species shows signs of progress and potential, such as creating AI or finding the mass relay network, the Reapers move into the galaxy and begin harvesting the 'dangerous' civilizations. This process was impeded by the Protheans the last cycle when they locked the Reapers out of the Citadel and therefore the rest of the galaxy. Since then, the Reapers have made various attempts to slow down galactic civilization while they approached the galaxy at normal FTL speeds. These attempts include the indoctrinating of the rachni, geth, and Saren. When encountered by Commander Shepard, the Catalyst represented itself using a hologram, taking the form of the young boy Shepard had watched die during the Reaper attack on 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. It then gives Shepard three options: destroy the Reapers, take control of them, or use the Crucible to create a synthesis of organic and synthetic life, thereby creating an entirely new life form.