This is not an official game. It's a fan-made modification or a ROM hack that incorporates the character Shadow the Hedgehog into the classic Sonic the Hedgehog game for the Sega Genesis. This version modifies the original game's data, replacing Sonic with Shadow and potentially altering other elements. Details regarding gameplay, features, and creators would depend on the specific ROM hack version.

Shadow the Hedgehog in Sonic 1

Shadow of the Beast - Mashou no Okite (Japan)