Catalyst

Background

The Leviathan DLC reveals that the Leviathans, who once controlled the Milky Way, created the Catalyst to break a cycle in which civilizations were destroyed by the synthetic life they made. Meant to preserve organic life at any cost, it concluded that conflict was inevitable and chose to harvest civilizations at their peak, absorbing their genetic material into new Reapers. It began with its own creators, turning the Leviathans into the first Reapers.

The Extended Cut DLC adds a fourth outcome if Shepard attacks or refuses the Catalyst, which results in a Reaper victory over the current cycle.

During Mass Effect 3 the Catalyst is first understood as a missing component of the Crucible; the Prothean VI Vendetta reveals that the Catalyst is the Citadel itself. When Shepard tries to activate the Crucible, Shepard is lifted to the Citadel's pinnacle, where the Catalyst explains its purpose and, conceding defeat, offers up to three options: Destroy all synthetic life, Control the Reapers by uploading Shepard's consciousness, or Synthesis, merging all life into organic-synthetic hybrids.