Continuing Eggman's Reign for his Sake
As part of his scheme to expand the Eggman Empire in his master's absence, Metal Sonic hatched a plan to use the power of the Master Emerald and the bio-data of Sonic the Hedgehog and Shadow the Hedgehog to transform in Master Overlord. Having already copied Sonic's bio-data from an encounter on the Egg Fleet, Neo Metal Sonic took the next step by having the Egg Fleet successfully invade Angel Island in Knuckles the Echidna's absence. After that, he simply laid in wait atop the Master Emerald until Sonic and Knuckles arrived, transforming into Super Neo Metal Sonic. He had both of them on the ropes, until Shadow eventually appeared and used his manipulation over Chaos energy to revert Neo Metal Sonic back out of his super form. Neo then copied Shadow's bio-data and used the Master Emerald's power to finally transform into Master Overlord. (StH: #7, #8, #9, #10)
After his transformation, Master Overlord started attacking Sonic, Knuckles, and Shadow at the shrine on Angel Island. Wanting to make them suffer, Master Overlord grabbed the trio and began flying towards the battleship that the heroes' allies were on so that they could watch him crush them. While he was busy dealing with the battleship however, Sonic, Knuckles, and Shadow escaped his grasp and began attacking his body. Master Overlord was further distracted when Sonic began taunting him, which gave Sonic's allies the opportunity to close in and start tearing Master Overlord apart. Just as he managed to shake everyone off of him, Master Overlord was rammed by the battleship that Tails was in control of. While he was distracted, Knuckles managed to pry the Master Emerald out of Master Overlord and return it to Angel Island. Without the Master Emerald, Master Overlord reverted back to his normal form as Metal Sonic. (StH: #11)
Master Overlord is a dragon-like entity of massive size, dwarfing even the Master Emerald's shrine. He has head armor similar to Neo Metal Sonic's, but with his quills combed more backwards, and he has two red eyes with black sclera aligned vertically on his face. Also, instead of a muzzle, he has a mouth with sharp teeth and red gums. He also has a long gray neck, a chest resembling a blue jet engine with a yellow and black intake, a thin gray spine for a torso section, and a blue pelvis that the Master Emerald is embedded in. Attached to his pelvis are four long, knife-like "legs". He also has a long purple and lavender-striped tail with two axe-like blades at the tip. Also, attached to both sides of his pelvis is a blue fin-like section, each one having three gray metallic wings with long knife-like feathers attached to them. He also has two arms, each of which is mostly made up of a gray cable with a thick blue gauntlet at the end. He also has four-fingered, round dark gray hands with knife-like gray fingers. Also, around each of his shoulders are two blue fin-like sections. Those fins closest to his torso also have three long gray spikes protruding upward from each of them.
As Master Overlord, Metal Sonic is implied to be even stronger than Super Neo Metal Sonic. In fact, it took the combined might of the Resistance's core members and allies to inflict noticable damage on him, and even this was not easy. In this form, Metal Sonic is capable of flight and possesses incredible strength, enough to stop an Egg Fleet battleship in midair by hand alone. He is also highly durable, being able to withstand the firepower of an Egg Fleet battleship without a scratch and multiple assaults from highly powerful individuals, although he would be slowly worn down by the latter. Even more so, he could survive having one of his arms blown off and most of his wings torn off. In addition, he could generate and shoot massive crystals as projectiles.