Who: Theta (the meta-crisis Doctor/10.5/whatever name you like)/Master
What: When two opposing forces meet...
Note: Just a quick piece to get it out of my head.
If there was one person that Theta Sigma could be said to 'fear' (as much as he was capable of feeling such), it was the Sovereign, also know as 'the Master' or 'Koschei' back in the real world. In the twisted reality of this so-called Pete's World, he was the Sovereign, a man who was also called World Ruler Harold Saxon. He had never lost power in this Earth despite how many he killed and this world's version of the Doctor (called the Traveler) never appeared to save humanity.
When Theta escaped from Pete's World and returned to the reality that had spawned him, he had been delighted to hear that the Master had died in a war against the Time Lords. It was one less thing to occasionally concern himself with. Instead, his lunacy focused fully on the Doctor and punishing this thing that had spawned him, that had left him the eternal weak shadow instead of a being in his own right.
That had been until a hand closed over his mouth and dragged him into the shadows on his favourite flea-market planet. His fighting and squirming had been short-lived as a device touched to the side of his throat and robbed him of consciousness.
When he awoke, he was on a bed with the Mas-... oh no, never that, always Koschei... leaning over him with a weapon pressed to his throat. Koschei who was supposedly dead and gone. The bleached out hair was truly unappealing Theta decided in the first few seconds.
“Who are you?”
“Theta Sigma,” he replied smugly, that grin already twisting his lips.
“That's not your name!” Koschei snarled, giving him a hard shake and a pain filled discharge from his laser screwdriver.
The next name that left the metacrisis' lips was the true name of the Doctor, one that only a few knew.
That sent the Master over the edge, a fatal discharge of energy killing Theta Sigma within seconds. Perhaps that was for the best since he didn't know the mutilation and kickings that his dead body was subjected to afterward.
Theta gasped in a breath as he sat up, hands clinging to the side of the metal table. It was almost worth coming back to see the utter shock on Koschei's face above him to have revived mid-autopsy. Death was swift to come again.
The third time he came to, Theta found himself back in the bed, stripped and chained down with a fully clothed Master looming above him. Where had he gotten that horrid hoodie? Not fitting if one asked Theta. Pity no one had.
“You can't die. You're like the Freak,” Koschei said with a smile that was horrible. “A Doctor that can't die. You're the hand, aren't you? A metacrisis that he didn't have the mercy to kill. All of you go mad, and you are too, an insane thing. He hid you in another dimension from me. How kind of him.”
Theta said nothing. This was the worst nightmare come true.
“Where shall we start?” Koschei asked with a cold laugh, the scalpel in his hand again. “A Doctor that will never die on me no matter what I do. Do you scream alike? I wonder.”
Theta soon proved that they did.