Arthur Pendragon was not an idiot.
Dragons were powerful. There was no way a slight scratch from his sword would be able to slay such a beast—yet Merlin insisted that was what had happened.
Which left only two possibilities. Number one: the dragon had just upped and left for no apparent reason. Number two: someone else had killed the dragon. And considering he and Merlin were the only two to return to Camelot alive, that meant that it had to have been Merlin.
Arthur wasn't sure which was more unlikely. But in either case, Merlin had lied to him, and Arthur couldn't fathom why.
No, he knew very well why, he was just still in shock from figuring it out. He preferred his blissful ignorance and longed to return to it.
The dragon would not have simply left, which meant Merlin must have defeated it—and there was only one possible way he could have done that.
Merlin was a dragon lord.
With that recognition everything made so much more sense. Why Merlin had been so distraught even at the dragon's very first attacks, before much damage had been done. Why he grew increasingly worn as the attacks continued. Why he had grieved so over Balinore—who in some sense was his kinsman, among the last that Merlin had.
Arthur could understand why his servant had kept it a secret. After all, the art of a dragon lord was far too close to sorcery for comfort in Camelot. The boy must have feared for his life. Arthur wouldn't betray his secret. He was just a dragon lord, after all, and now that the last dragon was dead, where was the harm in that? It's not like he was a sorcerer or something.
Merlin still didn't know that he knew. Arthur continued to go along with the charade, boasting to everyone that he had killed the last dragon! He hated himself for taking credit that was not his, but if it drew attention away from Merlin, so be it. The boy had saved Camelot—he deserved some peace.
But he couldn't let Merlin know he was being nice, of course. So during his "peace," Merlin was polishing his armor and sword, cleaning his chambers, mucking out the stables…the usual.
"For heaven's sake, can't you do anything right, Merlin?" Arthur asked him as the servant nearly dropped his sword. Arthur turned away to hide a smile.
One day he would tell Merlin he knew, but for now he was having too much fun.