The Perfect Scry (джен)

The world is falling into winter. Camelot is falling into war. Arthur will do anything he can to keep his kingdom safe, and he will use whatever weapons are at hand - but, to his horror, his decisions turn out to have severe consequences for Merlin. Naturally, Arthur decides that it is his duty to protect his loyal servant from the shocking truth...
“Let me see if I’ve got this straight,” Lancelot said, beginning to grin. “You want to teach Merlin to use magic – without letting him notice that he’s using magic?”
“He must not find out. If he finds out he has magic, he’ll – why are you laughing? This is no laughing matter, Lancelot!”
Set soon after Season 3.