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 Professor Gerald, helping him to create Shadow the Hedgehog. Black Doom then revealed that he would return to Mobius fifty years later to claim it as his own, for which purpose Shadow was modified to stand against him and the Black Arms. Doom's return was delayed, however, by his species' conflict with the Xorda.

Assisting Professor Gerald
When Professor Gerald was trying to complete his Ultimate Lifeform project to cure his granddaughter Maria Robotnik's Neuro-Immuno Deficiency Syndrome, he found barriers to his research he could not overcome without outside help. 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, granting Shadow his Chaos Control abilities. 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. (StH: #171)

Delay in Return
At some point during the fifty-year 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)

Background information

 * Black Doom was the main villain of the 2005 video game Shadow the Hedgehog, where it was revealed that Shadow was made from his DNA, making him Shadow's biological father. None of this has been confirmed in the Archieverse, though.
 * Due to issues with SEGA, Ian Flynn says that Black Doom and the Black Arms will not be making a proper appearance until they are given the green-light to have them defeat the Xorda and come to Mobius. (1)