Crucible

The Crucible is an ancient and highly complex device constructed by the Protheans as a superweapon to stop the Reapers, but never successfully implemented before their extinction. Its designs were stored in a Prothean archive on Mars and were later rediscovered by a Systems Alliance archaeological team. During the Reaper invasion of 2186, the designs are recovered by Commander Shepard on Admiral Steven Hackett's orders after the fall of Earth. The Crucible becomes the galaxy's last hope.

Design
It is unknown who initially began the development of the Crucible. Countless different species obtained and made contributions to the design over the course of millions of years, but none successfully deployed it before being wiped out by the Reapers. The latest species to try, the Protheans, were able to construct the Crucible, but before they could deploy it, infighting broke out between those who wanted to use it to destroy the Reapers and a faction that believed they could use it to control the Reapers. While the Protheans never had the chance to activate the Crucible, its schematics survived in a Prothean archive on Mars for the next 50,000 years.

Modern Construction
While examining the archives in 2186, Liara T'Soni found the Crucible's schematics and recognized their significance as a potential means to defeat the Reapers. Commander Shepard obtained them for the Alliance, which immediately began construction, a massive undertaking drawing on resources throughout the galaxy. Staggering financial costs were disregarded in the common effort to create something, anything, that could stop the Reapers. Despite the Crucible's elegant design, modern scientists could only determine that the device exploited the technology of mass relays, and were left to speculate on how it would ultimately function. More importantly, before the device could be activated it required one final component: the Catalyst. As revealed by the Prothean VI Vendetta, the Catalyst was the Citadel, the central control hub of the entire mass relay network. At some point in the past, contributors to the Crucible's design realized that it required a means to massively amplify its energy in order to be effective. The Citadel possessed this capability, and so was incorporated into the schematics.

Before the galaxy could combine the Crucible with the Catalyst, however, the Illusive Man fled to the Citadel and alerted the Reapers to the plan. The Reapers then moved the Citadel to the heart of their occupied territory, in orbit around Earth. With no other option, Admiral Hackett marshals the galaxy's fleets and proceeds to Sol with the Crucible.

Activation
The galaxy's forces distract the Reapers, allowing Commander Shepard to open the Citadel to dock the Crucible. Shepard then encounters the Catalyst itself, an entity that resides within the Citadel. The Catalyst explains that Shepard has three ways to use the Crucible: destroy the Reapers, control them, or use the Crucible to synthesize a new "DNA" for all denizens of the galaxy.

Regardless of the option the player chooses, the Crucible's firing causes collateral damage on the entire mass relay network. In the Destroy and Synthesis endings, the Crucible's use completely destroys the mass relay network. However, in the Control ending, the Crucible doesn't fire; instead, it releases a pulse that simply damages the mass relays and leaves the Citadel completely intact. If the player's EMS score is too low, further collateral damage can result from the Crucible's blast. See Priority: Earth for more detailed information.

Trivia

 * The classical definition of 'crucible' is 'a severe test, as of patience or belief; a trial'.
 * In The Art of the Mass Effect Universe, it is explained that the Crucible's design is meant to evoke an enormous, rocket-propelled bomb likened to a cross between a Trinity bomb and a NASA space probe.
 * The Crucible is visible as a hologram in the background of the Mass Effect 3 main menu.
 * If Mordin Solus joins the workforce constructing the Crucible, he sends a subsequent email to Commander Shepard describing the immense and complex project. He notes the human biblical story of the Tower of Babel as a metaphor for the co-operation between multiple species toward a single goal.