Hell Shadow is one of Shadow's Forms thats was born from his hate and pain.
Hell Shadow first appears when Sonic tries to revive Thrash from near death during Shadow's Cataclysm Spear. Hell Shadow is a result of Shadow's Hate of G.U.N and the pain of Maria's death fusing together to start an eternal flame in Shadow's heart, in by creating the Hell form.
Hell Shadow's Abilities
Hell Shadow has the Ability to Take over the minds of others and make them his slave, he can also, Like Shadow, can control Time and Space with the Chaos emeralds. When he has his hands on the Emeralds though he is an unstoppable Demon with no Remorse. Hell Shadow's Super form Is Hell Pyrosis Shadow, in this form he is (often) unbeatable.