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.
Below is a complete list of the different Black Arms encountered both in Shadow the Hedgehog and the Sonic the Hedgehog comic books.