Your worst enemy = your greatest friend (Merlin)

Your greatest enemie shall be your greatest friend

"One day, young king;
And I know you dislike magic.
I know what it has caused you.
And yet-
You will find the man who is everything you despise.
He - will reveal himself to be your strongest ally.
The greatest betrayal you shall ever receive,
shall have been for your cause and yours alone.
Your worst enemy
shall reveal himself your greatest friend."

This – this was the prophecy that was told to King Arthur himself by a druid.
How they encountered him was of no matter. The druid had come towards the king himself. It was of his own choosing that he revealed himself a threat to the kingdom.
And yet, there was no hate, no malice in his words as he approached the king of Camelot.
It was hope rather than fear that spread from him.
He didn't deserve to die though. Even the king agreed.

But Arthur could not safe everyone. In fact – the druid had given his life for Arthur.

However, these words, directed at "the once and future king", hang on in his mind for month's, if not years during his reign.
Years of doubts in his fathers lessons. Doubts in the outlawed magic.
Trust and mistrust in his allies and enemies.
And in the end, he wasn't sure whom to trust anymore. There were enemies he could trust. Friends he couldn't. There were only few he felt safe around.
The prophecy was spoken by a man whose prophecies were told to never be wrong.
A man who had foreseen his own death and greeted it like a friend.

So, this in mind, Arthur had made sure to check every single one of his allies. Just to see, if they were using magic in secret.
Because this, Arthur was sure about. The man who was everything he despised, must be a sorcerer.
However, King Arthur was not sure what to make of the whole betrayal for your cause thing.

Every knight was questioned, asked, and made sure to swear to never use magic.
Every royal advisor, every guest was interrogated. Kindly and in secret of course. No torture, no pressure. Some didn't even know, they were questioned.
Arthur was not as paranoid as his father. Especially, if the chances were that this sorcerer was on his side.

When the druid girl arrived, everybody believed that she were to be burned at the stake.
Instead, she was asked to meet with the king in person. And – alone.
No one. Not even Merlin was allowed in the throne room. Because Arthur had one question. And one question alone.

"You are Emala, daughter of the most famous seer of the druids. You must know who the sorcerer is. The traitor in our midst.", he asked. "When you answer me, you shall be free."
The druid's daughter looked at him with kind eyes.

"What if I don't tell you? He is a sorcerer. One of our kind. Why would I betray him."

Arthur frowned at her, as he walked around the empty room, thinking.

"If this sorcerer of yours is truly on my side. Then why are you protecting him? He is the one who betrayed you! You are my enemies." The question hung in the air for a while, as the druid daughter kept smiling to herself.

"You truly are the once and future king. You know nothing of your destiny.
I shall not tell you who it is and if it costs my life.
But know this: The druids call him Emrys. Emrys is the strongest and most powerful sorcerer who has ever lived.
Neither is he on our side, nor is he on yours.
We are on his side, just as you are. You just don't know it yet.

Magic, young king, is neither good nor evil. Magic is everything that surrounds you. It is in the heart of your people, in the life's of your man. Even within yourself.
Life is magic, and magic has given life to you. You know this.
Magic always has a price, but magic is also a gift. Nobody chooses to use magic. Magic chooses the one who wields it.
But the user can choose how to use it. And your friend has learned to use magic for you. It is you who decides whether that is a good or a bad thing."