The Black Arms Invasion was an event that occurred at some point in the recent past. The event saw the return of the alien race, the Black Arms, to Sonic's World after a 50 year cycle, leading to their attempt to consume and conquer the planet. The invasion was quickly brought to an end after the death of their leader, Black Doom, at the hands of Shadow the Hedgehog.
The Black Arms had been making preparations for the invasion as far back as 2'000 years ago, when the race transported several flying temples to the planet were they had remained becoming ancient ruins. (VG: ShtH)
During the Black Arms previous, and most recent visit to the planet, Black Doom met and became acquainted with Gerald Robotnik, who at the time was working aboard his Space Colony ARK to find a cure for the illness ailing his granddaughter Maria Robotnik. During this time, Black Doom agreed to aid the professor in his creation of the ultimate lifeform, Shadow the Hedgehog, to help further Gerald's research, as well as to help Black Doom in his goal to find the Chaos Emeralds, the latter giving his own blood to the creature. The partnership later disbanded when the professor realised the threat his new alley posed to the world. For this reason, Gerald planned to have Shadow and his newly built Eclipse Cannon super weapon to destroy the Black Comet upon it's next visit 50 years from then. This plan was initially forgotten after the professor sank into depression and became unhinged upon the death of Maria and his arrest at the hands of G.U.N. (VG: SA2, ShtH)
50 years later, the Black Comet returned to and immediatly began laying waste to the planet. It was at this moment that Black Doom came to Shadow and commanded him find the seven Chaos Emeralds for him "as promised". Shadow, who had been suffering from amnesia after almost losing his life from saving the planet from the falling Space Colony ARK, was clueless as to why Black Doom was asking him such a favour, but nonetheless concluded that doing as he asked would make the alien reveal the secret to his past.
The Black Arms continued to ravage the planet, as forces ranging from G.U.N. and the Eggman Empire to Sonic the Hedgehog and his allies made efforts to fight back. All the while, Shadow was retrieving the Emeralds, aiding any who needed his help in the process.
With the mysteries of his past finally laid to rest, Shadow went on to join G.U.N. under the leadership of Commander Abraham Tower, whom had finally relinquished his long-held hatred of the hedgehog. The planet suffered major demages that took an untold amount of time to fix.
The Black Arms were initially believed to be extinct after Shadow did away with them, but unbeknowst to anyone, the news of Black Doom's death had reached the Black Arms home world. With this, Black Doom's successor, Black Death (Black Arms), set off with a new army in a new Black Comet, with the intention of once again invading Sonic's world. Fortunately, G.U.N. were able to detect the new comet approaching, and Team Dark and the Spider Troupe were sent up to the comet to detonate a bomb that would wipe it out, nipping any potential invasion in the bud. The group succeeded in their endeavour, however, the creature known as Eclipse the Darkling was able to escape, along with the unhatched Dark Arms... (SU: #59, #60, #61, #62)