When Pushed to his limits, Legion's nanomite cells enter a state of hyper activity, growing and multiplying at super fast speeds, and perform as 100% efficiency, vastly improving his physical abilities by 1000% at least. Along with this enhanced physical states, he also produces the black smoke seen behind him, this smoke is in fact nanomite cells that are retreating off of his body in order not to cause him to grow out of control. Even though they are off of his body, they still act as a part of him and he can use them just like his other cells, the biggest difference is that he can now form them into wild weapons and tools that he would normally be unable to make.
However, this power has a terrible cost, it slowly chips away at his life (Cliche I know), the more he uses it and the longer he uses it the more of his life is chipped away, so he tries to avoid using it.
Story: Legion slammed into the wall behind him, as his assailant struck him in his solar plexus, and caused several ribs to crack. "Weakling" the assailant said spitting "When I found out what you were I thought you might be interesting to fight, but your hardly better than a normal human, what a waste." He approached Legion and grabbed him by the throat, 'Oh well, I'll just kill you and be done with it."
"Not so fast," Legion managed to say, despite the huge hands crushing his throat, with surprising speed for some one in his condition his right arm morphed into a knife with which he slashed his assailants arm. The assailant yelled as he dropped Legion who coughed up some blood before standing up. " YOu little Shit!" the assailant yelled "I'll kill you!" "No you won't." Legion retorted in disgust.
Without warning the wind around Legion began to pick up, and the air crackling with static electricity. Legion closed his eyes and held out his hands, and, to the assailant's surprise, was lifted into the air, and, with a burst of energy, a black smoke began to creep off of his form, his body bulked up slightly. After a single intense moment, Legion opened his eyes, both revealed to be robotic in nature now as opposed to only the one.
"Be proud of yourself," He said calmly, his would rapidly healing and his stamina increasing beyond measure, "Your one of the few to push me this far, pity you won't get to see it for more than a moment. With a flash he appeared behind the assailant "Full Force has been unleashed."