A/N: I thought up this premise a long time ago, but I only just now came up with a plot to go with it. So, enjoy.
I don't own Merlin.
Merlin always refused to share his waterskin with anyone. None of the knights ever really questioned it, they simply chalked it up as one of Merlin's many quirks. Merlin would insult Arthur, he would disappear for days at a time, he would survive battles without a scratch, and he never shared his waterskin.
And then one day, the inevitable happened. Merlin, Arthur and the knights were camped out on patrol and a thirsty Elyan reached for the nearest waterskin—in this case, Merlin's. "Don't drink that, Elyan," Merlin said sharply. "That's mine."
"You know, people don't die from sharing waterskins," Elyan joked.
"There's a first time for everything," Merlin replied, unamused. "Give it back."
"Haven't you ever heard of sharing with your friends?" Elyan teased.
"Give it here," Merlin insisted, trying to grab the waterskin. Elyan moved it out of Merlin's reach, then tossed it to Leon. The knights began playing a game of keep-away, throwing the waterskin to each other over Merlin's head.
After about a minute of this, Merlin sighed and sat down to stoke the fire. "When you get bored, please give that back," he said somewhat sullenly.
"I am bored, actually," Gwaine said cheerfully. "And I'm thirsty." He unstoppered the waterskin and brought it to his mouth.
"NO!" Merlin cried. Quick as a flash, he skirted the fire pit and snatched the waterskin out of Gwaine's hand. "You didn't drink any, did you?" he demanded anxiously. Before Gwaine could answer, Merlin spotted a dribble of water on his chin, and he whirled around to face the other knights. "Somebody grab my medical bag, NOW!"
"Merlin, I think you're forgetting who gives the orders," Arthur scolded.
"Merlin, I'm fi—" Gwaine started to say, but broke off in a coughing fit. Within moments, he began clutching his throat, struggling for air. The other knights stared in surprise.
"Do you want him to live?!" Merlin snapped. "Get my bag!"
Leon snapped back to his senses and obliged. Merlin guided Gwaine to the ground and propped him against a tree stump. Once Leon returned with the bag, Merlin quickly rummaged through it and pulled out an herb. "You need to swallow this, Gwaine."
Gwaine shook his head, gasping for air.
"I know it's hard, but you need to swallow this!" Merlin urged. "I will shove it down your throat if I have to!"
Gwaine looked rather taken aback by that suggestion. He took the herb and forced it down. A few moments later, he began retching. Merlin turned him to the side, just in time to prevent him from vomiting on his lap.
Once Gwaine was finished, Merlin brought out another herb. "Here. This will help your breathing."
This time, Gwaine took the herb without question. The other knights watched in tense silence. After several minutes, Gwaine finally managed to take a deep breath without either choking or coughing.
Merlin heaved a sigh of relief. "Good. You'll live. Just take it easy for the next day or two."
"Well," Arthur said somewhat frostily. "Would you care to explain why my knight nearly died from drinking your water just now?"
"Hemlock," Merlin said curtly, re-packing his bag. His hands were shaking.
The knights all shared a bewildered look of concern. "Merlin, are…are you trying to kill yourself?" Arthur asked worriedly.
"Quite the opposite, actually." Merlin closed his bag and turned to face them. "Do you remember the Mortaeus poison, a few months after I arrived in Camelot?" Arthur nodded. "Well, that was a very unpleasant experience, and I decided to do everything in my power to avoid repeating it. Ever since then, I've worked to build up and maintain a tolerance to any poison I can get my hands on. To that end, I routinely poison my own waterskin. And I've been increasing the dosages as time goes on. To anyone but me, that was a lethal dose of hemlock."
Shocked silence followed this revelation. No one was entirely sure what to make of it. It seemed like uncharacteristically paranoid behavior from their normally-cheerful friend…and yet, given what had just happened to Gwaine, it was clear that Merlin was deadly serious.
Finally, Leon broke the silence. "Good god, Merlin, why didn't you warn us?"
Merlin snorted humorlessly. "Would you have believed me? Or would you have thought I was joking, and perhaps even taken a drink to 'prove' it?"
Again, the knights were silent. None of them wanted to admit that Merlin was probably right. Merlin sighed and stoked the fire again.
This time, Arthur ultimately broke the silence. "Merlin…don't you think this is a bit excessive? Who would want to poison a servant?"
"You'd be surprised how many enemies I have," Merlin muttered darkly. "And my hemlock tolerance once came in handy."
"Hemlock and what else?" Percival asked curiously.
"I've got a high tolerance for aconite too. Still working on nightshade and foxglove."
"Remind me…never to use you…as a taster," Gwaine wheezed.
"Good point," Arthur agreed, cracking a smile. "Apparently there's a job where Merlin would be even more incompetent than he is at being a servant."
"Not dying is incompetence now?" Merlin responded, a glint of humor in his eyes.
"Yes, Merlin," Arthur said, completely deadpan. "You are now forbidden to move, breathe, or live."
Merlin pretended to consider this, then took a drink from his waterskin. "Hmm. Tempting, but I think I'll stick to being your servant. I can't tell you when you're being a prat if I'm dead."
