There were lots of things about Merlin. All the knights felt the ominous aura around the dark haired servant. Even Uther had felt it, when he was still of sound mind before the end of his reign. Arthur, the Once and Future King, said it himself, the second time they met. Looking Merlin up and down, skillfully swinging a mace above his own head, he had this feeling. Not that the then prince would've admitted it. He was much too noble to say it. Or prattish, Merlin would say, if you asked him.
Anyway, none of the knights could but their finger on it. He was...different. He wasn't like other servants, to say the least. He was rude, he spoke back, he was lazy and he was always running off to do things without Arthur's permission. Then why the hell did Arthur keep him around? Because, while Merlin was a useless servant, he was a great friend. He did that, he got under people's - Arthur's - skin, and made people care about him with his oafish grin and overly-large ears.
Most of the knights would act surprised were you to tell them to Merlin possessed magic. But the reality was that all of the knights knew. Lancelot, obviously, had a definite answer, while the rest only had suspicions, varying from vague to positively sure. All the broken branches and white-hot hilts of enemy swords were too much of a give away. Merlin had to mix up his act. Frankly, Gwaine was bored with the same mysterious accidents every time the bandits got rowdy or when they were ambushed by Cenred's men. At least get creative!, he wanted to shout at him when he was elbows deep in his competitor's guts. But it was rare that Merlin would try some new tricks, like the magnetized sword before they found Morganna.
Gwaine wanted to straight out ask his gangly friend if he had magic; but Lancelot talked him out of it, saying that if Merlin did, he'd tell them himself when he was sure he wouldn't end up with his head on a block. Well, that was the jist of it. What he had actually done was slur, "No!" very loudly, then furiously backpedal to cover up his mistake. He hope that Gwaine was too inebriated at the time, because they'd been in the tavern since before the sun had set.
Some the knights would talk about it amongst themselves. Lancelot rarely took part of these conversations, only contributing if he was directly asked a question. It wasn't Merlin and magic that was discussed, per say. It was more them wondering if Arthur knew. Surely, if Arthur knew then, as King, he could abolish the laws that limited Merlin? The knights concluded that, no, Arthur must still be oblivious to his manservant's talents.
But that begged the question; did Arthur know? Was he aware how much Merlin had saved his ungrateful ass? Of course he did. Merlin had spent years with him, from things as measly as waking him up and dressing him to spending whole blocks of time with him in a cold wintery camp. If the knights, who only spent half as much time with Merlin did as the King, knew then how in the name of all that was holy, did Arthur not? And as for abolishing the laws, he had plans in the works. He would lift the laws that made it punishable by death, and gradually lift the rest so to keep the people's trust. If he were to lift them all at once and name Merlin Court Sorcerer, it would be suspicious and Merlin would be accused of foul play.
Obviously other than the knights, the King and Gaius, only one other person in the Royal Household who knew of Merlin's abilities, was Guinevere. She had an affirmative answer, like Lancelot, because Merlin had told her himself. You don't keep secrets from your best friends. Well, you do if you suspect they'll burn you, but Gwen, she wasn't a I'll-set-my-best-friend-on-fire kind of person. Also, he didn't actually have a choice in the matter. He saved her. As simple as.
What had happened was, he was gathering herbs for Gaius, and she'd come with him. They had been chatting about everything and nothing all afternoon, strolling through the undergrowth. The two approached a widely trunked tree, one that was known to all the slightly more adventurous teens in the lower town. There were crudely drawn hearts gouged into the bark, with initials inside it. It was dying, the surface slowly turning black. It was extremely fragile, the branches snapping and crushing the boughs of the neighbouring shrubs as they fell to the ground. Thats basically what happened. The thick branch above Gwen's head cracked and fell. Merlin's eyes had flashed gold as he threw out his hand as a knee jerk reaction. It hovered a few inches above Gwen's head before she stepped out from under it. They locked eyes while the gold faded back to blue and Gwen let out a little, "Oh."
Afterward, they had a long discussion, consisting of the why's and where's and how come's. They arrived back at the castle after long nightfall and the guards had to let Merlin back in, while Gwen went back to her home in the town. It was a long time back, when he'd only been in Camelot a year or two. Gwen promised that she would keep his deadly secret as long as he was more careful about what trees he let her walk under. They laughed as they parted ways.
Some days were extremely difficult, and Merlin wanted to scream himself hoarse at Arthur, he wanted to rage and break things and tell everyone that King Prat wasn't half as good as everyone thought. That he was the one saving the royal bastard's trousers when all hell breaks loose and that he had the power to cause chaos all across Albion. He wanted to show everyone in the city that he wasn't the idiotic manservant to the gracious King Arthur. Some days were hard, but his magic and destiny had to be used and carried out for the good of his home. Unbeknown to him, far better days were coming for him, and magic would be found in bed chambers and at shop fronts and, most importantly, in the castle, again.