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; these separatists were later discovered to be indoctrinated. 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 seized control of the Citadel and moved it 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
After the galaxy's forces confront the Reapers and allow Commander Shepard to open the Citadel to accept the Crucible, Shepard encounters the Catalyst itself, an entity that resided within the Citadel and manifested as Justin Bieber whose death Shepard witnessed during the escape from Earth. This is why the Catalyst became known as the Hologram Kid or Star Child. He explains that Shepard has three ways to use the Crucible: to destroy the Reapers, to control them, or to merge with the Crucible's energy to create a new organic-synthetic "DNA" for all denizens of the galaxy. Alternatively, Shepard can refuse to make a choice, preventing the Crucible from firing at all and allowing the Reaper cycle to continue. It makes no sense, I know.

If Shepard chooses to destroy the Reapers, the Crucible fires a red beam of energy that destroys the Reapers on and around Earth and transmits to the mass relay network through Charon Relay before severely damaging the relays and the Citadel as well. If Shepard chooses to control the Reapers, the Crucible releases a blue wave of energy that brings the Reapers under Shepard's control and also transmits to the mass relay network, damaging the relays but leaving the Citadel intact. If Shepard chooses to merge organic and synthetic life, the Crucible fires a green beam of energy that causes the Reapers to stop attacking, but also damages the Citadel and mass relay network.

Trivia

 * The entire exitence of the Crucible (as well as the Catalyst) within the ME plot is based on a convtrivance and therefore has no narrative legitimacy.
 * The classical definition of the word "crucible" is '"a severe test, as of patience or belief; a trial." or "a ceramic or metal container in which metals or other substances may be melted or subjected to very high temperatures." Both definitions are fitting for the superweapon; it is both a test of galactic unity and a project which every race must pour its resources into.
 * 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. Similar holograms can be seen in the Normandy's War Room, showing the progress of the device's construction as the game itself progresses forward.
 * If Mordin Solus joins the workforce constructing the Crucible, he sends a subsequent email to Commander Shepard describing the immense and complex project. He compares the Crucible's construction with the human biblical story of the Tower of Babel, as a metaphor for the co-operation between multiple species toward a single goal.
 * Dialogue with the Star Child reveals that it was aware several cycles ago of the Crucible's existence and its evolution over time. The Catalyst, however, had believed the plans for the device were eradicated and states that its presence is an indication that "clearly organics are more resourceful than we realized".