Crucible

The Crucible is the ancient superweapon that becomes the galaxy's last hope against the Reapers in Mass Effect 3. Countless species contributed to its design over millions of years without ever deploying it; the Protheans built one but fell to infighting between those who wanted to destroy the Reapers and an indoctrinated faction that wanted to control them, and the schematics survived in the Prothean archive on Mars for 50,000 years. In 2186, after the fall of Earth, Liara T'Soni finds the plans in the Mars archive, Commander Shepard recovers them from the Cerberus infiltrator Eva Coré, and the Alliance begins construction while Shepard recruits support from other species. Completing it requires the Catalyst, and at the end Shepard is offered up to three ways to activate it: destroy all synthetic life, control the Reapers, or merge organic and synthetic life through synthesis.