The Shadow Dimension, also called the Shadow World, is a location that appears in the Sonic the Comic series published by Fleetway Editions. It is a separate dimension from Mobius. The dimension is maintained by means of the Black Emerald.
The Shadow Dimension is populated by humanoid alien shadow creatures, who are unable to live in the (positive) dimension of Mobius in their natural forms. They developed a plan to conquer Mobius. First, the shadow called Shayde made contact with Mobius through a temporarily split in time and space, and encountered Johnny Lightfoot. As a shadow, Shayde was able to become an exact double of Johnny by making contact with him. Johnny was sucked into the Shadow Dimension and Shayde took his place on Mobius, infiltrating the Freedom Fighters but also allying himself with Doctor Ivo Robotnik as Agent X.
Agent X used Robotnik's resources to secretly build a matter transporter on Flickies' Island. When the Freedom Fighters attacked Flickies' Island, Agent X laid a trap for them and sent Sonic to the Shadow Dimension with the matter transporter. He planned for a shadow to take Sonic's form and return to Mobius as his evil double - a fate that Agent X planned for all the other Freedom Fighters as well. However, a bespectacled shadow accidentally revealed to Sonic the existence of the Black Emerald, and Sonic promptly smashed it, causing the entire Shadow Dimension to be destroyed.
Sonic and Johnny were returned to Mobius when Tekno the Canary reversed the settings on the matter transporter. When the real Johnny made contact with Agent X, X was changed back into a shadow and sent back to the (self-destructing) Shadow Dimension.