Speculation on how the inevitable conversation/confrontation could go.
“I am not my father.” The words burst out of Arthur, angry, full of betrayal and grief and fear.
“I know you’re not.” Merlin’s reply is quiet, thrumming with the underlying confidence and faith and trust that has always been there, even when the words are prat, and arrogant, and condescending.
“You lied to me. All this time—“
“And when exactly, sire, was I supposed to tell you?” Merlin interrupted, throwing up his hands. “My very first day I came to Camelot, before I even set foot into the castle, I witness a beheading of a man who had used sorcery. Two weeks later, his grieving mother nearly assassinates you using magic, and I’m the only one who can—and does—stop it the same way. And then along comes Valiant, with his bloody giant magical snakes coming out of his shield, for crying out loud. Then the Affanc in the water, and the poison in the cup, only I suppose that was more an attempt on my life rather than yours…” He trailed off, distracted, and then shook his head. “Not to mention the unicorns, and griffons, and Questing Beasts, and trolls and Sidhe and all the other bloody sorcerers who attack you every other week, either for a personal grievance or because they’re looking to strike back at your father by killing the son! So you tell me, Arthur. When and how exactly I was supposed to tell you that I am a warlock? That I was born this way, with magic in my veins, just as you were?”
Arthur sucked in a harsh breath and opened his mouth to reply, but Merlin wasn’t finished.
“So yes. I lied to you, by omission and directly. But I did so to protect others who were innocent and unable to protect themselves, and a couple of times, I lied to protect you from yourself.” He blew out a long breath and met Arthur’s eyes defiantly. “You are not your father, but you are your father’s son. And it would have killed you to murder Uther out of rage. I would not see that scar on anyone’s soul, no matter what the sins of the father may be. No matter what the sacrifice.”
His last sentence is quiet, but carries the loudest into the space between them.
“As always, sire,” and this time there is no hint of irony or sarcasm in his voice or movements as he bows, chains rattling slightly.“I am at your service, and your mercy.” He stepped back into the cell, away from the door.