Crucible

Mass Effect 3
The Crucible is massive, highly-complex device that appears in Mass Effect 3.

While examining the Prothean archives on Mars, Liara T'Soni found schematics for a weapon of incredible scale and power, strong enough to even defeat the Reapers. It appears that the Protheans tried to construct the device as a last hope to defeat the Reapers, but were unsuccessful in finding the "Catalyst", a vital component to create the machine. After protecting the blueprints from Cerberus, Commander Shepard sent the design to Alliance Command and construction began immediately. Though some express skepticism regarding the Crucible's function, Admiral Hackett makes a shrewd comparison to the atom bomb.

Although massive in scale, its design is described as "elegant": Giant, but simple.

As revealed by the Prothean VI Vendetta, the Crucible wasn't originally designed by the Protheans, but was rather the work of other species throughout the cycles of extinction stretching back millions of years, improving upon it. The Protheans were close to deploying it when a group of separatists thought to control the Reapers rather than destroy them.

The final piece for the Crucible was called the Catalyst, the Reaper's master AI that was revealed to be the Citadel, which the Crucible attached to. The Catalyst itself manifested as a young human boy Shepard saw killed by the Reapers during the escape from Earth. After explaining the origins and purpose of the Reapers, the Catalyst gave Shepard three options: control the Reapers, destroy them, or merge with the energy beam to create a new organic-synthetic "DNA" for all denizens of the galaxy. In all scenarios the Citadel emits a Blue/Red/Green wave that hits Earth representing Shepard's choice of either Control/Destroy/Synthesis.

In Destroy/Synthesis the Crucible also fires a Red/Green beam at the Charon Relay, destroying both the Citadel and all the other Relays.

In Control the Citadel closes up and survives as the Reapers leave Earth, and the damage to Charon is not as massive as in the other endings. Indicating a possibility for the Relays to survive in this ending.

However, if Shepard's "Effective Military Strength" score is low enough that the player is on track to the worst ending, the Catalyst will not offer a choice; the Catalyst will automatically assume that Shepard either wants to destroy the Reapers (if Shepard destroyed the Collector Base during the suicide mission) or wants to control the Reapers (if Shepard preserved the Collector Base during the suicide mission). No "merge" option is provided.

If the decision to destroy the Reapers is made with a low Effective Military Strength, the resulting blast wave will decimate Earth along with the Reapers. To avoid this, make sure you have a decently high Effective Military Strength.

If the Control decision is made with a low Effective Military Strength, the blast will cause a massive amount of damage on the planet but will not wipe out all life.

Trivia

 * 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.