The Black Comet is a comet that passes the Earth every fifty years. It is where Black Doom commands his army, the Black Arms, and appears in the game Shadow the Hedgehog. The levels Final Haunt, Black Comet, and The Last Way are all set in the interior of the Black Comet. The comet was destroyed by Shadow the Hedgehog at the end of the game, bringing end to the race of Black Arms (excluding Shadow himself).
The comet itself appears to be alive or techno-organic in nature, most notably when it was teleported to Earth and seemed to take on a life of its own and dig into the surface. When inside, it is almost like being in the body of a living organism. There are pulsating growths and what appear to be tentacles, organs and throat-like halls and many organic sounds. For example, in the Japanese version of the game, when the tentacles are destroyed, a roaring similar to the Biolizard's can be heard. There is a viscous substance in the comet that only death leeches and worms can exist in. Even Black Doom continuously warns Shadow that "that toxic substance is fatal". The tentacle-like projections also appear to exist in the substance.
A comet is a small solar system body made of ice and dust. The Black Comet, however, does not appear to made of ice. The Black Comet is more likely an asteroid.
The last time the Black Comet passed by Earth, Black Doom made a pact with Gerald Robotnik in order to create the Ultimate Lifeform, Shadow the Hedgehog. Black Doom would provide the genetic material Gerald need to create Shadow, and Shadow would in turn help Black Doom in his quest to collect the seven Chaos Emeralds. Gerald, however, did not trust Black Doom and suspected his plot to obtain the emeralds. As such, Gerald designed the Eclipse Cannon to destroy the Black Comet when it returned, before he was incarcerated on Prison Island by G.U.N.
Shadow the Hedgehog
Fifty years later, during Shadow the Hedgehog, the Black Comet once again approached Earth and the Black Arms began the final phase of their mission to collect the seven Chaos Emeralds and enslave the humans. Since the Black Comet did not have enough speed to make it to Earth's surface on its own, Black Doom used Shadow's current memory loss against him, tricking him into finding the seven Chaos Emeralds.
Once Shadow brought the Emeralds to Black Doom, the alien overlord used Chaos Control to teleport the Black Comet down to Earth, where it released a paralyzing nerve gas that coated the planet. As Sonic, his friends and Eggman remained frozen, ready to be eaten by a horde of Death Leech, Black Doom explained how the Black Arms would keep them and the humans like livestock, using them as slaves and a food source. However, Black Doom's plans were foiled when Shadow used the Emeralds to turn into Super Shadow and destroyed Black Doom, before teleporting the Black Come back into space where the Eclipse Cannon destroyed the Black Comet and all of the Black Arms inside of it.
In the Pure Hero level Final Haunt, the Black Comet is (predictably) full of Black Arms. However, on the Semi-Dark final level Black Comet, it's full of G.U.N. robots, and there are few (if any) Black Arms present.