The Death Egg Saga.
Dr. Ivo Robotnik, having suffered many failures at the hands of Sonic the Hedgehog and his friends, built a weapon the likes of which no one on Mobius had ever seen before: an egg-shaped flying fortress he dubbed "The Death Egg". Due to its immense size, The Death Egg required vast amounts of power to keep it operational, namely power rings, which Robotnik managed to get his hands on by using the vessel's massive intake tube to drain all of the remaining power rings from a unique grotto on the outskirts of Robotropolis.
However, the Swatbot overseeing the intake tubes' operation failed to shut the ingestion process down as it pulled the tube from the grotto and ended up pulling a great deal of debris as well as Sonic's faithful sidekick Tails into the vessel, setting Robotnik's schedule back due to the time it would take to separate the rings from the debris.
Once collected, the rings were spread across the vessel in a chain-mail like pattern to render the vessel invulnerable to external assaults, which would prove useful until Robotnik could ascertain that no more rings were in the debris and dump the extra weight, which Tails later did after acquiring a set of Swatbot armor to keep his presence hidden from Robotnik and his armies.
However, even with the debris gone and the rings set in place, The Death Egg was not yet fully operational. In order to bring it to full power, Robotnik required a Chaos Emerald and set his sights on the twin chaos emeralds hidden deep under a volcano on Angel Island. Upon its arrival, The Death Egg slammed into the face of the volcano and started drowning Angel Island into the sea.
But before Robotnik could claim the emeralds, Tails destroyed the master control panels and not only tore The Death Egg off of Angel Island but sent it flying into space. Tails later rigged it to explode and when it did, the fury of the explosion was felt and seen all across Mobius.
The Second Death Egg
Before he suffered a mental breakdown, Robotnik's successor, an alternate reality Robotnik called Eggman, had been building up his EggDome Bunker at the heart of New Mobotropolis for reasons that Sonic and the others were unable to figure out. During the fall of the Iron Dominion's reign over his Empire, Eggman had regained his sanity and went into hiding with Snively, Lien-Da and the Dark Egg Legion.
During this time, slave laborers starting building docking ports and refueling stations all across Mobius for something massive. Sonic and the others were certain that Eggman was behind it but had no such proof. While construction of the ports, stations and the Death Egg itself were underway Eggman and Snively traveled to the Special Zone and took a Chaos Emerald from Feist. A Chaos Emerald that would change Mobius forever.
When Sonic and the others least expected it, the completed Death Egg rose into the Heavens, shedding its EggDome "flesh" to reveal its true form. Sonic along with the demented wizard Ixis Naugus and the Freedom Fighters attempted to bring the flying fortress down but could not harm it save for one spot through which Sonic and Sally entered Eggman's new toy. What happened next changed everything.
While distracting Sonic with a new and improved Silver Sonic, Eggman killed Sally in cold blood using remote guns and then channeled the full power of his new Chaos Emerald through The Death Egg and unleashed a reality altering wave of Chaos Energy. The Second Death Egg was erased from existence once the new timeline settled in place.
Death Egg III
The Death Egg III, christened the Wily Egg, was created with the combined efforts of Dr. Eggman and Dr. Wily.