This takes place at the alternative timeline of Sonic 3 & Knuckles while Sonic was fighting Robotnik alone at Angel Island. Robotnik managed to get a good hit on him, causing Sonic to fall into the depths below. That's when the Grimeware virus struck. It consumed him from the inside leaving a hollow husk of a vessel for Grimeware to take control of. It seeks to find and feed off of other victims one by one in brutal and horrific ways to add to its hivemind.