Black Doom

Black Doom is the evil leader of the Black Arms alien race, a species of parasites that travel from star system to star system in search of conquest. Fifty years prior to the current war on Mobius against Dr. Eggman, Black Doom made contact with Gerald Robotnik, helping him to create Shadow the Hedgehog. Black Doom then revealed that he would return to Mobius fifty yeras later to claim it as his own, for which purpose Shadow was modified to stand against him and the Black Arms. Doom's return has been delayed, however, by his species' conflict with the Xorda.

50 Years Ago/Delay
When Gerald Robotnik was trying to complete his Ultimate Lifeform project, he found himself unable to do it with the materials on hand. Making a deal with Black Doom, he extracted some of Doom's blood to help him finish the process. The result was Shadow the Hedgehog, the Ultimate Lifeform with the abilities granted by Gerald's process and also with a measure of Doom's own power. Black Doom afterwards departed, vowing that he would make his return fifty years later, prompting Gerald to modify Shadow so that when the time came Mobius would have him for a protector. However, at some point during the interlude, Black Doom entered his people into a war with the Xorda, which has devastated the Bem homeworld of Argentium and likely many other planets and star systems. (StH: #170-171)