The Black Arms are a malevolent race of marauding aliens in the Sonic the Hedgehog universe. Appearing as the primary antagonistic force in Shadow the Hedgehog, they invaded Earth with the intention of harvesting all human life so that they could consume humans as livestock. They have devastated numerous worlds in this manner and would have done the same to Earth had it not been for the efforts of Shadow the Hedgehog, who defeated the Black Arms' leader and destroyed their vessel/homeworld, the Black Comet.
Shadow the Hedgehog
Not much is known about the Black Arms from before their invasion of Earth, though it is known that their leader Black Doom played a vital part in the creation of Shadow the Hedgehog, the ultimate life-form engineered by Prof. Gerald Robotnik. Over fifty years after Shadow's creation, the Black Arms returned to Earth and launched a full-scale attack. Their goal was to obtain the seven Chaos Emeralds, magical gemstones containing limitless power, so that they may might use the gems' power to achieve their objective of abducting Earth's human population all at once, then consume them.
During the Black Arms' invasion, Black Doom made contact with the wayward Shadow, commanding him to find the Chaos Emeralds. Shadow, having previously lost his memories and sense of purpose, went in search of the Emeralds in order to obtain the answers he desperately sought. Over the course of Shadow the Hedgehog, Shadow came into conflict with the armed forces of the United Federation, the Guardian Units of Nations (G.U.N.), Sonic the Hedgehog and his friends, the robot armies of Dr. Eggman and even fought against the alien invaders themselves. It was only after he obtained all of the Chaos Emeralds and brought them to the Black Comet that he learned the truth: Professor Gerald had created him using the blood of Black Doom. Black Doom intended to use Shadow as his instrument of conquest, but Professor Gerald wanted Shadow to stand in defense of the Earth. Though conflicted and devastated by this revelation, Shadow decided that he would protect the Earth and he used the Chaos Emeralds' power to defeat the Black Arms. Becoming Super Shadow, he defeated Devil Doom and used his Chaos Control ability to warp the Black Comet back into space. Shadow then destroyed the comet using the Eclipse Cannon aboard the Space Colony ARK, wiping out the Black Arms completely.
Archie Comics' Sonic Universe
In the Archie Comics' Post-Super Genesis Wave continuity, it is revealed that the Black Arms had not been entirely wiped out. Another Black Comet invaded Sonic's world, this time led by Black Death, who attempted to finish what Black Doom had started. G.U.N. sent an armed force to infiltrate the Comet and destroy it with a nuclear device before it could make planet-fall. Accompanying this task force was Team Dark, consisting of Shadow the Hedgehog, Rouge the Bat and the rogue Super Badnik E-123 Omega.
Team Dark attempted to plant the nuke in the centre of the Comet while the rest of the G.U.N. unit fought the Black Arms, but Shadow was confronted by Black Death and Eclipse the Darkling. The two managed to brainwash Shadow and turn him against his allies, but this did not last as Rouge and Omega managed to snap him out of it. An enraged Shadow confronted Black Death while Rouge saw to the deployment of the bomb. Team Dark ultimately succeeded as Black Death was killed and the New Black Comet was destroyed. However, Eclipse had escaped to the planet below and vowed to take revenge against Shadow.
Below is a complete list of the different Black Arms encountered both in Shadow the Hedgehog and the Sonic the Hedgehog comic books.