The Xorda Attack on Mobius was an event that took place some time during Mobius' recent history by the alien race known as the Xorda. The aliens' attack was a result of the discovery that their previous attack had failed to wipe out all intelligent life on the planet. The Xorda had located a group of Overlanders in space and, realizing they were very similar to humans, returned to the planet to finish the job they started thousands of years before.
Thousands of years ago, the Xorda were antagonized by the human population on the planet Earth. As a result, the Xorda bombarded the planet with Gene Bombs in hopes of extinguishing all intelligent life on the planet (i.e. humans), while leaving the other living creatures alone. The Gene Bombs already had been killing all humans. The Gene Bombs also sped up the evolution of the animal kingdom, blending their DNA with humans. All of this, however, was unknown to the Xorda, who had already withdrawn. (StH: #124)
The secrecy of continued intelligent life on Mobius (as Earth came to be known) was compromised when the Xorda discovered two ships heading for Mobius. They discovered that there were Overlanders aboard and attacked their ships. They traced the ships to Mobius and realized that intelligent life had arisen again. Upon entering the Solar System, they nearly stumbled upon a ship from the Bem, a much more benevolent alien race, but were distracted by Shadow the Hedgehog. When they arrived, they delivered an ultimatum of destruction unto the people of Mobius, and also told them the truth about their world's origins. As if to demonstrate their power, they launched a devastating single-ship attack on Dr. Eggman's forces. (StH: #124)
The Xorda attack ship managed to cause significant damage to Dr. Eggman's city. However, the ship was repulsed by Giga-Bot Prime, a giant robot created by Dr. Eggman that only Sonic's speed could power. However, the Xorda had yet to play their trump card. (StH: #125)
In the planet's polar region, the Xorda revealed their final plan to eradicate the planet (they no longer cared to preserve non-intelligent life). A pyramidal device, known as the Quantum Dial, was set up. Its purpose was to tear open a hole in reality, creating a black hole powerful enough to consume the planet and the entire solar system. It was assumed that such a large target would be easy to deal with; given that, Station Square launched an immediate missile strike. However, the Quantum Dial possessed some kind of field that caused any construct within fifty feet to fall apart. Unwilling to give up without a fight, the various fighters of Mobius banded together, putting aside their differences to battle a common enemy. However, even with the amazing number of warriors aligned against them, the Xorda weapon was only vanquished after the timely resurrection of Knuckles the Echidna (who simultaneously caused the destruction of the weapon's defenses) and the seeming suicide attack of Sonic the Hedgehog upon the Quantum Dial, which caused the destruction of the device and the disappearance of Sonic. The Xorda were unaware, having departed Mobius soon after they launched the Quantum Dial, leaving the entire planet for dead. (StH: #125)
As a result of Sonic's disappearance, Dr. Eggman formed the Eggman Empire, a massive political body with hordes of machines as its military. He then proceeded to take over Angel Island with the assistance of the Dingoes, under the new leadership of Kage Von Stryker. Additionally he managed to completely destroy the Golden Hive Colony. The Eggman Empire nearly succeeded in launching a full-out nuclear strike on both Station Square and the Kingdom of Knothole. This was prevented by the timely return from outer space of Sonic the Hedgehog and an immediate joint-strike by the Knothole Freedom Fighters and G.U.N.. However, the political divisions of Mobius continued to favour Dr. Eggman since the Xorda's attack for some time to come. (StH: #130, #131, #132, #133, #138, #139, #140, #175)