A/N: So, I don't own the Keys to the Kingdom (not like you didn't know that). Anyway, this takes place just before the Will is executed and Arthur has wrested the Seventh Key from Sunday.
Arthur watched as the Seventh Key left Sunday and floated around it. The moment it touched his skin, all of his doubts about himself instantly evaporated to be replaced by an unyielding confidence.
Yes, he thought to himself. I've won. There's nothing that anyway can do now, I've wrested all of the Seven Keys.
Sunday refused to look at him, still remaining haughty as usual, but he didn't matter now. Not when the Seventh Key was around Arthur's neck.
He stood and moved to use its power, but hesitated. There was something that weighed in on his mind, something that was small, but like most of the aches of the mind had become larger and larger until he could hold it in no longer.
"Tell me something," Arthur said to the previous Lord of the Universe. "And answer truthfully, otherwise I'll have the answers torn from you using the Seventh Key."
"If you want to know about the Will, you'll find out soon enough," Lord Sunday said, once again not looking at Arthur, as if he was still somehow above Arthur and replying was beneath his station.
"No," Arthur said. "It's about when you had held me captive in that replica clock prison and those puppets were about to bore my eyes out… why did you ask them to stop?"
Sunday sneered. "Isn't it obvious? You told it so yourself, it was because doing that would have been equivalent to taking it by force."
"I thought so," Arthur said. "But I knew that that wasn't completely true. For some reason, you stopped before doing that. If you hadn't, I don't know how long I would have lasted. You could have kept me imprisoned for a long time… but you didn't. Why?"
"I considered myself above such measures," Sunday replied, and Arthur instinctively knew he was lying.
"Answer me truthfully," Arthur said while touching the Seventh Key. He could feel Sunday fighting it, which was odd, considering the fact that he hadn't seen Sunday actually fight at all until now.
Sunday couldn't resist though, not with Arthur holding all three Keys.
"I couldn't bear to harm you," Sunday said through clenched teeth.
"Why?" Arthur asked. There was a long pause as he realized that Sunday was still trying to resist, and did not want to answer this.
"Why not simply kill me instead and be over with it? It would be a mercy in many ways," Sunday said.
For a moment, that seemed to be a very nice suggestion to Arthur who felt that it would be an excellent display of his powers as he destroyed over half the Incomparable Gardens in a single swipe… but something in him stopped that.
"You still haven't answered," Arthur said, infuriated now. "Answer!"
"I couldn't harm you like that," Sunday said. "I couldn't, because you are my son."
That was enough to throw Arthur completely off balance. Sunday was lying, there was no way that that could be true.
And yet, when Arthur pressed him using all of his Keys again, he got the same response.
"There's no way that could be true," Arthur said. "My parents are Bill and Emily Penhalingon."
"Not your real parents, now, are they?" Sunday asked. "In truth Arthur, you are our child. I hid you away and placed you with two people I knew would love you like their own, because I never wanted all of this for you."
"Our? Who's my mother?" Arthur asked.
"Saturday," Sunday replied. "Don't look so shocked, she was the first Denizen, and so was different from the later ones my mother created. Saturday was modeled differently, to be more flexible than the others. Yes, some time after the breaking of the Will I began visiting her more frequently, and we began meeting more. A few hundred years later, we visited the Earth once. The idea for a child was her idea initially, but at the time for some reason I felt that the idea was a good one, probably due to this… this… mortal side of me."
"But she's a Denizen!" Arthur shouted.
"Indeed," Sunday said. "And also an accomplished sorcerer who oversaw my birth and that of my brothers. She made a vessel for herself, quite like the story of the Minotaur's birth on Earth, and then you were born. And, for some unknown reason, for all that we could tell you were mortal. Completely mortal."
"It was then that I came to my senses," Sunday. "I couldn't have a child born. There was no way that we were ready to raise a child, and a child would have been weakness. The side of me that was closest to a Denizen woke up and I realized my mistake. Not only that, but my mortal side also realized that you would never be happy as my son. It was then while we were still on Earth that our presence created a disease, one that afflicted you. One that lead to your lung disease."
"My parents died in that!" Arthur protested.
"And your adoptive parents were easily tricked into believing it, think about it Arthur, Saturday could have easily wiped their memories of you, making a few fake ones wasn't hard for me. Though of course, I hid it from her," Sunday said.
"You see, Saturday couldn't let you go. I told her when you were sick that you couldn't possibly be cured using the Keys, or go to the House, because it would be interference. I lied to her, and told her that you died. And she's hated me ever since then, she blames me for being weak and letting her firstborn, her only son die. She broke all ties with me and has attempted to defeat me ever since," Sunday said.
"Did the Will know?" Arthur asked.
"I don't think so, at least I don't believe it knew in its mind," Sunday said. "I do find it almost impossible to believe however, that you being chosen as the Rightful Heir was an accident. It might have been inexplicably drawn to you, but I do not believe that it knew. When I was about to order those puppets to finish the deed, I found that I could not, even though it was the right thing to do. I think for some reason, your Denizen side was completely hidden."
"But why-" Arthur was interrupted as the Piper and Saturday landed near the hill.
The Piper tried to coax the Keys off of Arthur, but Arthur resisted. The Piper then tried to kill Saturday, but Arthur sensed it before he could complete the action. Arthur also bound the Piper so that he couldn't kill himself, because Arthur wasn't going to be losing any more relatives.
And then the rest was history. The Will was complete, the Universe destroyed, and then rebuilt.
As the New Architect split himself and sent the mortal Arthur off towards Earth, He now knew everything said by Sunday to be true. After all, He had had the Atlas and knew everything in it.
He also knew that becoming a Denizen for him, was, unbelievably, a sure thing even if he hadn't been caught up with the Will and used the Keys. Sunday had been right about the fact that his Denizen side had been suppressed, which he know knew about thanks to the Atlas. If Arthur had lived till the age of fourteen, he would have started turning into a Denizen anyway and would have completed his transformation by the age of twenty-five anyway. It was pure chance, or maybe something else, that he had been chosen before that had happened.
He also knew that there was something in the Elysium, a secret alcove under the rock that Lord Sunday knew about.
The New Architect visited the rock, and easily flicked it over though he had given the Seventh Key to Suzy. Inside, he saw a small old-fashioned photo album. Inside were a collection of photographs. There was him when he had been little, his fourth birthday party, him playing on the xylophone, all things that Lord Sunday had managed to capture and place in this album.
He did care, the New Architect realized. The New Architect also knew that Sunday, Saturday, and the Piper, who was his uncle, could all be brought back. Though he knew that no one could replace Emily, perhaps it was time though to spend a little while with some other parents.
A/N: So, there are a lot of things unexplained in the Keys to the Kingdom, and one thing is why Lord Sunday didn't continue onwards with boring out Arthur's eyes when he could have (it is also suggested that it wasn't the case that taking them like that wouldn't have been taking them by force) and also that we never were told much about Arthur's biological parents. Plus, whatever the Will's criteria were for choosing him in the first book seemed a little vague (the First Part said it didn't want someone too young or too old) but was that really just it?