The Most Powerful Machine in the Universe

Dr. Demented strode in front of me. He threw down Lucy's head. I didn't see the rest of her body.
I didn't panic. It was not too late to save her. I had access to all the knowledge of the Archives. I could rebuild her. There was a copy of her memories in the New Archivist's diadem. Hopefully my future self had neglected that, in his dementia. I could save her. I could rebuild her. But first, I needed to avenge her.
A machine teleported into Doctor Demented's armor. It performed a quick scan, and teleported out a microsecond later, reporting it's data. I launched more and more probes, and gathered more and more data. I assembled it into a complete picture of the armor I would one day wear, the weapons I could one day create, and the mind and body that would one day be mine.
I needed to work quickly, since I was racing against a more powerful version of myself. More machines teleported in. The brought the Time Key out with them.
I had a primitive system in place to deal with the cosmic technology. I began to communicate with it.

It was a strange reversal for me. I thought so fast. So much faster than those around me. I could plan out a thousand variations of every response. But the Time Key was faster still. It thought and lived a billion times shorter than I.
Who are you, It asked.
A past version of Dr. Demented.
What do you want?
What can you do? Can you fix him?
No.
Can you make it so he never exists?
No.
Can you freeze him. Pause him. Let me work in peace for days, years, millennia.
Yes. What will you do for me, human?
I am not a human. What do you want me to do for you?
I want to die.
After I cure Dr. Demented, I will determine how to kill you.
No. You will determine how to destroy me first.
I was reluctant. I didn't want to give up such a powerful artifact. But there was always a chance that I could create another one. Okay. Here is the deal. You freeze Dr. Demented in time. I remembered Lucy. And you preserve my friend there. Then. you bring the my whole mansion to some barren moon in the distant past. You bring us all back when I am done.
You are forgetting something.
I wasn't used to forgetting things. Because I was used to quintuple-checking all of my thoughts. You won't be able to bring me back. You'll be dead.
It will be a one-way trip for you. You will be embarking on a project that will take eons, When you return, the Earth you knew will have changed beyond recognition.
I considered. If you just leave Dr. Demented frozen, could we call it square?
No. You must destroy me.
What if I say 'no'?
Then I unfreeze him.
I considered. Could you take us all back in time, so I finish in the present day?
Going backwards in time is painful. I will not do it again.
A wormhole? Or could you dilate time so it only takes a second in Earth's reference frame?
Both of those are beyond my capabilities.
Really? Are you sure?
Of course I'm sure.
I thought some more. If I accepted the Time Key's deal, I might well be able to save Dr. Demented. Save myself. That was good, Invaluable. But I would lose Earth. Lose everything I had ever known. I considered.