The Black Comet is a comet that passes the Earth every 50 years. It is where Black Doom commands his army, 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 comet passed by Earth, Black Doom came down to Earth and made a pact with Gerald Robotnik, Dr. Eggman's grandfather, in order to create the Ultimate Lifeform, Shadow the Hedgehog. Shadow would help Gerald in his research and would help Black Doom in his quest to collect the seven Chaos Emeralds. Gerald, however, does not trust Black Doom and suspects his plot to obtain the emeralds. So Gerald designs the Eclipse Cannon to destroy the Black Comet when it returns before he is then thrown into Prison Island by G.U.N. 50 years later, as the Black Comet approaches Earth during the start of Shadow the Hedgehog, the aliens begin the final phase of their mission to collect the 7 Chaos Emeralds and enslave the humans. The Black Comet does not have enough speed to make it to Earth's surface on its own however, so Black Doom uses Shadow's loss of memory against him and tricks him into finding the seven chaos emeralds. Once Shadow brings them to Black Doom, he reveals his plan and Chaos Controls (teleports) the Black Comet to Earth. The comet then releases a paralyzing nerve gas that quickly coated the planet, turning the world red. As Sonic, his friends and Eggman remained frozen, ready to be drained of blood by a horde of Death Leeches, Black Doom explains how the Black Arms will keep and breed them along with all the humans like livestock, and use them as a slave and food source. However, Black Doom's plans are foiled when Shadow uses the emeralds to turn into Super Shadow, destroys Devil Doom (Black Doom evolved), teleports it back out into space and uses the Eclipse Cannon to destroy 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.