Merlin didn't exercise this part of him nearly enough. He'd thought since his knew it was there, it would just always capable of being drawn on- and it was, but he was vaguely aware of a headache building behind his eyes.
The sky cleared. The comet came into view in all its glory, and the clouds blew into wisps. As Merlin let go of the magic, it was like braking from a hard sprint, and as his eyes cleared back to normal, he staggered slightly, a sweat beading on his brow.
"Done," he said, satisfied with the results, but he had to bend over and lean his hands on his knees with an exhausted groan. "Oh wow. That- that is hard work. I think I got stitch."