The Eclipse Cannon is a giant laser cannon that is an ultimate weapon of mass destruction designed by Professor Gerald Robotnik on board the Space Colony ARK, able to destroy planets and pierce stars.
It had been deactivated for a long time. The cannon could operate only when the 7 Chaos Emeralds were added to it. When 6 of the Emeralds were found, Doctor Eggman demonstrated its power as the "might" of his tyrannical empire, by blowing up half of the Earth's moon. The cannon can be seen on the exterior bottom of the ARK as it forms the "nose" of the research center.
Despite its frightening power, the intended purpose of the Eclipse Cannon was to destroy the Black Comet in order to save the Earth from being overrun by the evil alien forces known as the Black Arms residing there.
Sonic Adventure 2In Sonic Adventure 2, the Eclipse Cannon was first introduced to Doctor Eggman by Shadow the Hedgehog to grant his one wish of world conquest. Shadow explained that all seven Chaos Emeralds were needed to fully power the weapon. The two of them, with the government spy Rouge the Bat, who was sent there to gather information about Shadow, gathered a total of six Emeralds (one Eggman already had, one Rouge offered to join up, the one that Shadow stole, and three stolen from Prison Island). Eggman then used the Cannon to destroy half of the moon to terrorize Earth into submission. However the demonstration proved to have flaws, such as the cannon requiring a full day to recharge due to not possessing all of the Chaos Emeralds. Eggman also made a video conference call with the President, where he supplied the latter with the ultimatum of either surrendering the United Federation or else he will ensure that every single major city in the United Federation, if not the country itself, will be wiped out by the Eclipse Cannon. The genius madman also planned on giving the President 24 hours to decide whether to submit to Eggman, although Sonic's arrival at the President's limousine a second after this announcement, as well as Sonic bluntly refusing, forced Eggman to withdraw the ultimatum. With the cannon requiring 24 hours to charge due to not having all the Emeralds, Sonic and his friends were able to penetrate the ARK and stop the Cannon before it fired again.
Eggman, who was apparently unaware of what happened, installed the last Emerald he took from Tails. A hologram of Eggman's grandfather Gerald Robotnik appeared, which revealed that he secretly downloaded a doomsday device that activated when the seven Chaos Emeralds were brought together as part of his vendetta against the G.U.N. government for the death of his beloved granddaughter, Dr. Eggman's late cousin Maria Robotnik. With the immense energy being moved to the Cannon's Core, the ARK was sent on a collision course to Earth. When Knuckles and Sonic bring the Master Emerald to the core, it turns out to be a shrine similar to the one on Angel Island, designed to maximize the emeralds' power. Shadow defeated the Biolizard, and they managed to control the Chaos Emeralds. Despite this, the ARK was still crashing to Earth. Eggman revealed that the prototype had used Chaos Control to attach itself to the exterior of the ARK, pulling it towards the planet.
Shadow and Sonic used the Chaos Emeralds to transform into their Super States. They fought and defeated the "Finalhazard", as it fell to Earth. Though, in order to stop the ARK from falling, the two used Chaos Control to bring it away from the planet's atmosphere. Due to exhausting his powers by teleporting the space colony, Shadow crashed to Earth. Sonic returned to the control room with his ring and told everyone what happened. As they all left the ARK, Sonic looked back and said, "Sayonara, Shadow the Hedgehog.". At this point, Sonic believed this was Shadow's death.
Shadow the Hedgehog
In Shadow the Hedgehog, after the events of Sonic Adventure 2, the Eclipse Cannon was apparently repaired by G.U.N. (Presumably, they also removed the Gerald Robotnik program from the Eclipse Cannon's main control system so as to not allow history to repeat itself, as evidenced by Shadow later using the seven Chaos Emeralds to destroy the Black Comet shortly after teleporting it out of Earth's range). The military also deployed robots to guard the control room. During a possible Dark Side storyline, the Eclipse Cannon and the ARK were later to become under control of Black Doom and the Black Arms (with the assistance of Shadow), who wanted to use the cannon to take control of Earth. With five Emeralds, the cannon destroyed the White House and Central City. During the Last Story when Shadow was fighting Black Doom, a hologram of Gerald revealed to Shadow that the Cannon was developed to destroy the Black Comet. The Cannon's purpose was fulfilled after Black Doom was defeated by Super Shadow. Shadow used Chaos Control to teleport the comet into outer space and fired the Cannon to destroy the comet.