Arthur had nearly gotten killed five times already, Morgana was becoming one relentless pain in the neck, the knights were worn down from the day-long battle, and Merlin himself was so fed up from constantly having to save them all that he was ready to scream. He ducked as one of Morgana's men moved to stab him, and Arthur knocked the sword from their hand and used the hilt of his own to strike them unconscious. He turned to Merlin, smirking.
"Why don't you actually start doing something, Merlin?" he quipped, before running off to help Percival.
Merlin's eye twitched. Although he couldn't tell, they also began to glow a bright gold.
Thankfully, Morgana was too far away to notice, to discover who he really was, but Arthur wasn't, and the moment the tree nearby exploded in a rush of bark and leaves and soaring branches, Arthur turned to see if Merlin was alright. Only, he saw that Merlin was completely unharmed, eyes vivid. Arthur's face paled, and Merlin, too angry to realize what he'd just done, continued to stand, angrily defiant, blasting the nearby men away several feet. After he'd calmed down, the battlefield around him was mostly silent, and when he saw Arthur's expression, he could have sworn that his heart stopped beating.
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