He led them into a small hut, built into the edge of the forest. Rory gasped as he went inside, but to Amy things that were bigger on the inside seemed commonplace, especially in the recent days.
"Doctor, you have a chameleon arch, correct?"
"Not something that I use anymore, but yes."
"Doctor, what's a chameleon arch?" Amy asked, shivering at the sundry collection of helmets, weapons, shields, guns, and other tidbits and battle trophies this boy had collected.
"A biological rewriter."
"Excuse me?" Rory asked, knowing not what the Doctor meant.
"Here's an example, companion of the Doctor," the little boy, the "Sentinel" said. "If your friend the Doctor used a chameleon arch, he would change. He would become a different species. His essence would be contained in a small watch, which he would never notice. If he opened the fob watch, his essence would be released, and he would be Time Lord once more."
"Who are you?"
"Your childhood friend."
The doctor's face darkened. "You died. 900 years ago."
"A chameleon arch may preserve its victim, as it may revert the subject back to a child."
"Who are you?" Amy asked tentatively.
"An ancient enemy. And an old friend." The Doctor said.
"The king of All. Welcome back to . . . The Master!" He cackled.
"How aren't you a time lord?" The Doctor said.
"Defective chameleon arch. Restored my memories, but not my form. My immortality is different."
A muffled roaring came from outside.
"Excuse me a moment," The Master said. "The Shakri have become rather Relentless these days. Be careful not to break anything."
"So who is this . . . Master?" Amy asked, after The Master had left.
"A childhood friend of mine. We were inseparable, until the time came for him to look into the untempered schism. He was driven insane, and was forever plagued with an endless, repeating rhythm. four drum beats, repeating over and over. He died at the end of my Tenth incarnation, or so I thought."
"Rory! don't touch that!" The Doctor yelled.
"Who are the Shakri?" Amy asked the Doctor.
"Mythical beings, stories told to keep Gallifreyan children in their place. They are also an alien species. But that thing out there . . . That was no Shakri." The doctor said. There was silence for about an hour, besides The Doctor telling Rory off a couple more times.
"Oi! why so silent? Wouldn't want such a gloomy atmosphere in my house!" The Master was back, looking posistively lively even through the covering of blood over his sword, and his whole left side.
"One more thing." The Doctor said. "How did you end up on Trenzalore?"
BOOM! Plot Twist. Just Know that this is before the Doctor knows that Trenzalore is his "Grave."