It was a bandit attack. Because of course it was- it seemed like Merlin couldn't go an hour in the forest without being attacked by some ragtag group of bandits. With Arthur and the knights, the time seemed to be cut in half.
Everything was going as per usual: the knights were engaged in lots of flashy intense swordplay with the bandits, and Merlin was hiding behind a tree. Well, he was also helping subtly by causing bandits to trip over roots and rocks, or by causing tree branches to fall on their heads. Unfortunately, he was so engrossed watching the main fight that he failed to notice the bandit sneaking up behind him until there was a blade sticking two inches out of his chest. For a moment, there was no pain, just confusion, because surely that couldn't have just happened.
But then the blade was pulled out and the pain hit, and it felt like his chest was on fire. Suddenly there was blood, and a lot of it, and Merlin's head felt like it was stuffed full of cotton. Unable to stand, he fell onto the ground, barely feeling the impact. The world swam with black, and Merlin's last thought was that he needed to warn Arthur. And then there was nothing.
A while later when the knights had dispatched the last of the bandits, Gwaine looked up and noticed that Merlin was missing. As he looked around, he felt something tug at his gut, and for a moment, the breeze seemed to pick up.
But then it was gone, and when he found the servant, Merlin was propped up against a tree, rubbing the area over his heart, a bloodstained knife nearby. "Fighting's over," Gwaine informed him.
"Arthur?" Merlin asked
Gwaine offered him a hand, "Princess is fine, just looking for you."
With a groan, Merlin accepted the outstretched arm and joined the knight, "Well let's go find him."
Gwaine didn't even think to ask about the knife.