Black Comet/Pre-SGW

The Black Comet was a celestial object and the mobile homeworld of the Black Arms led by Black Doom. The comet is capable of traversing the galaxy, allowing the Black Arms to attack and consume the populations of worlds they come across. The comet approached Mobius fifty years ago, and Black Doom contacted Professor Gerald Robotnik and aided him Project Shadow, with Doom planning to use Shadow the Hedgehog as an agent.

History
The comet departed soon thereafter, and while on its return trip to Mobius the Black Arms ran afoul of the Xorda, another alien race with its own claim on Mobius. The two races began warring, with the Black Comet currently in orbit around the ravaged Argentium. (StH: #169, #170, #171; CSE)