When Mysterio apparently died once more (in reality he was trapped in the Ultimate universe) his suit was placed on the market by a unknown individual (implied to be Hobgoblin) and was sold to a anonymous person who then used it to rob banks by using it and the reputation that Beck had garnered to scare people off, dubbing himself Mysterion. This was successful until he was confronted by the Superior Spider-Man, who quickly seen through his old tricks and tried to catch him, pursuing him throughout New York while The Punisher went after him, presuming him to be the original Mysterio. Even with the assistance of Hobgoblin behind him, he was eventually caught by Octavius and trapped with the rest of his former Sinister Six allies.
Mysterion was created by Christopher Yost and David López, first appearing in Avenging Spider-Man #22 - Pro-Death.
Major Story Arcs
As apart of Octavius's collection, he would be implanted with a mind control device and deployed as apart of his "Superior Six": his old group of villains now "reformed" and helping Superior Spider-Man out against his foes. Octavius didn't care much about the well-being of his subjects, but did this to give himself a good reputation and reinforcements.
After a encounter with Sun-Girl, Octavius's brainwashing tech was disabled, leaving his Superior Six free to go after him. While most of the members were angered and willing to murder Ock for his actions against them and violating their freedom, Mysterion was scared and just wanted to leave without getting hurt. When another fight took place before the group could kill Octavius, Mysterion begged Sun Girl to let him escape, and she allowed him to do so out of pity.
Mysterion resurfaced attacking New York with a army of robotic dinosaurs before he was defeated by a group of superheroes very quickly.
Ever since the real Mysterio returned from his banishment and became a public villain again, Mysterion vanished and hasn't been seen since. It is possible that he was either recruited as a assistant for Beck, much like Francis Krum and Daniel Berkhart became, or has retired from crime altogether.
Powers and Abilities
Mysterion had all of the gadgets and capabilities that came with the original Mysterio, but as noted by the Superior Spider-Man, had no actual skill or nuance in using them, choking on his own gas and creating illusions that barely fooled anyone who had their wits around them. He had no fighting skill either, struggling to escape the likes of the Punisher and almost dying to a gunshot.
While it seemed like he had some technical knowledge (being able to use the suit in the first place, creating robotic dinosaurs, etc) he wasn't nearly as talented as Beck was.