Shadow’s origin in Sonic the Hedgehog 3 teases the introduction of a major franchise villain in a future movie. Sonic 3 wasted no time introducing Shadow the Hedgehog, one of the most popular characters from the Sonic video games. In many ways, Shadow is the primary focus of Sonic the Hedgehog 3. The story of Sonic the Hedgehog 3 follows Sonic, Tails, and Knuckles, who are tasked with taking down Shadow. However, they lose their initial encounter against Shadow, which leads the Black Blur to reunite with Gerald Robotnik, the grandfather of Ivo Robotnik.

https://screenrant.com/sonic-the-hedgehog-3-shadow-black-doom-setup-op-ed/