Black Arms

The Black Arms are a violent race of parasitic aliens who travel the galaxy in which Mobius is found seeking conquest. Led by their sinister leader, Black Doom, they have been to the planet of Mobius itself at least once in the past, and were set to return there fifty years after that first recorded visit and the creation of Shadow the Hedgehog. However, they became embroiled in an interstellar war with another equally violent race, the Xorda, and thus were unable to return to Mobius for a time. The conflict has presumably affected several worlds, the only known one being the Bem homeworld of Argentium.

50 Years Ago
Visiting Mobius fifty years prior to their conflict with the Xorda, the Black Arms activities in this period are unknown. All that is known is that they had partial contact with Gerald Robotnik, the primary scientist aboard Space Colony ARK, and that he used Black Arms genetic material-namely that of Black Doom himself-to create Shadow the Hedgehog, his Ultimate LIfeform. Learning that the Black Arms would return to Mobius in fifty years to conquer it, Gerald modified Shadow from his original purpose-that of developing a cure for Gerald's ill granddaughter, Maria-to that of defending Mobius from the Black Arms upon their return. Shadow later learned this from Gerald's digital diary, and joined the Guardian Unit of Nations to prepare for their eventual return. (StH: #171, #182)

War with the Xorda
The explanation for the delay in the Black Arms' return to Mobius was uncovered by Sonic the Hedgehog, Knuckles the Echidna, Miles "Tails" Prower, and Merlin Prower when they traveled to Argentium to rescue Amadeus Prower and his wife Rosemary, Tails' parents. At some point, the Black Arms had become involved in a war with the Xorda alien race, and the resources diverted for the conflict prevented the Black Arms from following through with their plans for some time. (StH: #170)