Mecha Shadow is a robotic version of Shadow, similiar to Mecha Sonic, created by Dr. Eggman (Eggman Nega in the Sonic Speedball RPG series) capable of using Chaos Control and other Chaos Emerald-based abilities, even without having a Chaos Emerald.
He appears as the final boss of Sonic's world created by the Dimension Eaters Version/Units XJ-&^7623542378378621@ to destroy what has began. He was broke down and seen broken in the Dimension Junction .
Metal Shadow/Mecha Shadow apears in ULTRA Sonic the Hedgehog (also known as Sonic the Hedgehog 5) as Super and Hyper's enemy. Every time he gets an Ultra Emerald, he gets stronger, until he gets all 7 of them (witch turns him into Mecha Shadow). It is obvious that he works for Dr. Eggman.