Black Doom took this form after the truth behind Shadow's creation was revealed. As the Black Comet was invading Earth, Shadow used the Chaos Emeralds to transform into Super Shadow and fought Devil Doom. Once hit with enough Chaos Spears, Devil Doom fell to his death, expressing shock at how a supreme being and immortal life form could be defeated.
Devil Doom is the final boss of the game. He attacks by breathing fire, raising rubble and sometimes throwing it at Shadow, and strange rocks which shoot lasers, either revolving around him and shooting vertically, or circled around him and shooting outward.
To defeat him, Super Shadow must use Chaos Spear on his eye. The best way to do this is to charge Chaos Spear until Shadow says "Chaos", then get near Devil Doom's eye and fire. Whenever it is hit, Doom's Eye will move from one of Devil Doom's two heads to the other. Shadow must find which head has the eye on it and attack it. If any rubble or orbs are between Shadow and Devil Doom, Shadow is likely to attack them instead of him.
Sometimes Devil Doom will teleport using Chaos Control, which he always does if Shadow gets close enough to dash into him. When he reappears, he will often raise rubble. Among this rubble are balloons, which contain rings to help Shadow maintain his Super form. Dashing into him to make him teleport and then raise the rubble with balloons is a good way to obtain more rings if Shadow is running low.
If the fight goes on for long enough, the gas from the Black Comet will enter Earth's atmosphere and paralyze everyone below (though this does not affect the battle, and there does not seem to be a time limit). Just over eight minutes in, Eggman will confess that Shadow is not an android (as he would claim in some paths Shadow can take in the game), but the same Shadow from Sonic Adventure 2, and that he had one of his robots rescue him from the fall. Near ten minutes, Devil Doom will do an evil laugh at Earth being taken over by the Black Comet, although the battle will still go on.