Alexander Star wondered how he had come to be. He knew that he had once been a different person, and his Crucible had judged a new version to be superior. So he had been regenerated into a a younger, more clever individual. But could he regenerate into something else? Could he regenerate into someone taller? Someone with a tattoo on his arm? Someone with claws.
Alexander decided to find out. So he ripped of his thumb. The flesh boiled and rippled as a new thumb formed. No claw. He tore off the thumb again. No claw.
He spent hours mutilating himself, trying to manipulate his body. It hurt, but pain was just sensory input to him at this point. It didn't mean anything to someone who could recover from any injury in a fraction of a second.
Alex looked with some disgust at the growing pile of severed limbs and shorn-off fingers. He realized that the pile probably weighed more than he did. That was disgusting.
Alex knew he should stop injuring himself. But think of all he could do with this new power. He could make himself look like anything.
He was just about to stop (his third 'last try' in a row) when Dr. Demented entered his room. "What is this," he demanded. He wasn't wearing his armor. His musculature bulged through his futuristic robes, making him look like a strange combination of an octogenarian, a body builder, and someone at a science fiction convention.
"I wanted to see if I could control how I healed."
"And so mutilate your own flesh and blood?"
"Well... yes. I couldn't think of a better way to do it."
The Doctor stared at Alex for a moment. Then he smiled. "Curiosity. Good. And no harm done to one who can heal as you. Unfortunately, you is not understand working of Crucible. Does not create what you want. Creates what is."
The Doctor summoned a sentence of coherent English. "If you want something of the Crucible, tell the Crucible you lost it."
Alex thought he understood that sentence. He needed to act as if he really did have claws. He needed to convince the Crucible he had claws. He ripped off a hand. Didn't work.
Once more. It worked! Yes!
Alex flexed his serrated hand. He sliced through a steel table like a hot knife through nothing.
Did he even need to injure himself at all. What if he just thought that his normal state was to look like, say, a dragon? He concentrated. Slowly and surely, he began to change.
The Master of Time was working out some of the finer details of his master plan when a gigantic dragon burst into his laboratory. "Foolish boy," the Doctor said. "To alter his own brain. What he do without me, eh?"
He was already wearing his armor, because his armor knew it was going to be needed (the Time Key was an unreliable messenger through time, but it was vastly better than nothing). He froze all of his machinery in time, so it wouldn't be damaged by the rampaging dragon. A quick scan revealed the location of the Crucible.
"What am I doing," the Doctor asked. It wasn't a rhetorical question. He had completely forgotten his own battle. His armor reminded him that he had just been looking for Alex's heart. Dr. Demented cursed his forgetfulness, and formulated a plan. He plunged his hand into the dragon's chest. The energy of a small atomic bomb was expended tearing apart the dragons flesh. The Doctor wrapped his fingers around the Crucible, and locked them into position. The flesh of the dragon attempted to expel the armor, but nothing could break the armored grasp of Dr. Demented.
All the air in the laboratory was turned to plasma. The Crucible put more and more energy into destroying or expelling the foreign body. Meanwhile, the Doctor was attempting to control the flow of time near the Crucible. It was difficult, made even more difficult by the fact that the Crucible was a doorway to a whole other dimension. With quite a lot of work, the Doctor reversed then flow of time for the Crucible. The Crucible now thought that Alex was the host's natural state, and that the dragon was an alteration. The dragon evaporated, and Alex was reassembled from the subatomic level up.
From Alex's point of view he had suddenly gone from willing himself to be a dragon to reconstituting from being... a dragon. "What happened."
"Dragon not sentient."
Alex tried to ask for more details, but the scientist ignored him. Machines came back to life, and the Doctor did his work.
Alex wondered what else he could do. Carefully keeping his brain the same, he grew into a twenty meter tall giant. He created and retracted claws. He made lasers come out of his eyes and fingers.
Wait a second. If laser beam were coming out of his fingertips, did that mean he had little bits of electronics in his body. A few minutes later, he had a smartphone screen on his arm.
But, hold on. Alex extruded a virtual reality console from his hand. He tore off the console, and found it still working. He could create anything he wanted. This was going to be fun.