Only the vaguest part of Arthur's brain was working properly. Merlin. His best friend. Using magic. Sorcery. Dragons! It went against everything his father had eradicated from Camelot! Just thinking about the situation made Arthur numb. Being king came with a lot of difficulties, but none that were this significant. The cold, calculating and logical side of his brain (Arthur called this the Uther side) told him that Merlin must be arrested, tried and executed. Since Uther's death, sorcerers were still being executed, but it was fairer. They were being given fair trails. There was no more executing of innocent peasants.
Merlin would be found guilty; if the King decided to accuse him, there would be nothing he could say, fair trial or not. However, the just, caring and kind part of his heart told him to at least speak to Merlin about this. He has the right to explain himself. Arthur trusted Merlin with his life. He knew every inch of his best friend, and he wasn't a bad person, magic or no magic.

The golden walls of Camelot Castle burned with a feverish flame. The setting sun had turned everything blood red. A crow landed on to a window ledge. Arthur had told Merlin to meet him on the battlements at sundown. Knowing the servant, he would be late.
"You asked to see me, sire?" Merlin's familiar voice came from behind him.
"What really happened at the caves, Merlin?" He spoke with the confidence of someone used to getting his way. He should have known better.
"Oh, um, nothing! Really! I just distracted the dragon and saved your sorry neck! Like I told you, I have many talents!" It was only now that Arthur saw what he meant by that. And wondered how he could have been fooled by such a bad liar.
Arthur tried not to laugh. Instead, he sighed in frustration. "Merlin, I need the truth. The dragon…what were you saying to it?" The warlock's blood froze.
"I have no idea what to tell you. Giaus had always said I wasn't careful enough with it…" he trailed off, uncertain where to begin.
"With what?" asked Arthur. He needed to know. He thought he did know, but needed to hear it from his best friend.
"Magic" whispered Merlin. He said it as though now that he had revealed his secret, his gift would leave him.
Arthur looked impassive. "Explain," he asked in a flat voice.
The Warlock looked at the king; he started his story.
"I was born with magic. Mother told me to always keep it a secret. I could do so many things! But Mother never liked it when I used magic. So I never did. As I got older, it became more difficult to hide. So Will found out – "
"You used Magic to save us! And took no credit? Why would you…Oh." Arthur trailed off. His Father.
"Yes. I would have been killed." Merlin was speaking in a monotone. If Arthur was going to kill him, he didn't see what difference it made tell Arthur all of this.
The king stared at him. Having to work in a castle with Uther Pendragon…Merlin was actually braver than himself!
Finally Arthur spoke. "You saved us that day." That wasn't a question. "When else have you used magic to save us?" Thinking back, there seemed to be lots of times there, seemingly, luck was on their side. Now Arthur wasn't sure.
So Merlin told him everything. Leaving nothing out.
"You!" Arthur cried out in anguish, pain and anger. "You killed my father!"
"No, I –" Merlin was cut off by a punch to the side of the head.
"Everything that happened…Because of you!" Arthur was in shock, shouting, tears streaming down his face. His father was killed by a sorcerer. Merlin was that sorcerer. Though he was so outraged, so radically furious, his heart told him Merlin would never do it deliberately. It urged him to calm down, to listen. A fierce battle raged inside of him. His face was pained, the tears still falling. Merlin backed against the wall. Watching. He could have done something, but he realised it wouldn't have worked. It would just have made everything worse. So he remained on silent, for the first time in his life. As slowly as the sun was setting, Arthur leashed the anguish and turmoil back into his heart.
One single question burned brighter than the flames of the torches that were now lit.
"What happened?"
Merlin stood up, shakily. The answer to the question would determine weather he would live or die. One name was all it took.
"Morgana."
The name dripped with evil.
Arthur fell to his knees. The sound of him howling pierced the night. Merlin could do no more than watch while Arthur sobbed and raged and cried out in so much pain that it hurt Merlin to listen to.
It was deep into the night when Arthur suddenly stopped. He was tired. It was too much for any normal person to deal with. But he was the king. He was no normal person. Now knowing what he did, it was up to him to decide if Merlin should be tried or not. And he already knew what he was going to do.
"Merlin, you have lied to me, your king and friend. You have hidden your sorcery for many years. As you know, sorcery is a most heinous crime." Arthur had had a trying day. But he couldn't do anything to harm his friend. And there was the bit Merlin had told him about being Emrys, the most powerful sorcerer. And the whole 'Two to the same coin bit thing…
Arthur gave him a watery smile. "But I am not going to execute you. As shocking as it is, you are my only friend. I will deny that if asked," Merlin snorted. "But from what you have told me, you have used you magic to protect me and my knights. And saved Camelot from many threats. I can't believe you would have thought to hide it from me! I understand when my Father was alive…and the pawn you were in killing him. But you don't have to hide anything from me." Arthur was slightly embarrassed and formal. It wasn't often he gave out compliments.
Suddenly Merlin grinned. He wasn't going to be killed!
"Well, in that case, you are a clot pole! I was worried about you! Are you alright?" The concern the warlock had for the King was genuine, while the insult was not.
"I think so. And just because you may have magic, and are still alive, doesn't mean you can insult your King. Have fun in the stocks!"
And, despite all they had been through in that one night, things went back to normal.