I thought that Merlin letting Morgana give him this Fomorrah was a ridiculous premise, as Merlin would never let even possible hurt come to Arthur for any reason. This took hold of me while re-watching the episode.
It may continue, it may not. But here it is for your enjoyment anyways. Starts off during the episode 'A Servant of Two Masters.'
"Fomorroh are creatures of dark magic." Merlin didn't like the sound of that. Dark magic, so dark even Morgana called it dark magic. This was decidedly one of the worst situations Merlin had been in in a long time. "Even if you cut off their heads, you cannot kill them. Another will just grow in its place." Merlin looked at Morgana with trepidation as she walked closer. "In the days of the old religion they were revered by the High Priestesses, as they allowed them to control people's minds." Oh shit. "Fomorroh will suck the life force out of you. Everything that makes you Merlin will be gone. And in its place will be one thought."
Didn't take a genius to figure out where she was going with that - I'm going to have you assassinate Arthur! Morgana was a dangerous enemy but very predictable. More so dangerous was the Fomorroh. He could take his chances with the Fomorroh and hope Gaius could stop him, or break out right now and reveal himself to Morgana. "One thought that will grow until it's consumed you completely. One thought that will be your life's work. You will not be able to rest until it's done." If the Fomorroh took control of his magic, there is no way anybody would be able to stop him, and Merlin would never forgive himself. Well, that decides that. "And that one thought is simple. You must kill Arthur Pendragon."
Merlin went limp as he shouted "Onbindan!" [untie] He fell to the floor as Morgana gasped, wrenching his hand free of the ties. He scrambled around on the floor, shouting "Ådlþracu gegrind!" [force of impact] The shelf next to Morgana fell on top of her, giving Merlin time to escape the hut. He scrambled to his feet and shakily launched himself through the hut door. He got about 50 feet outside of Morgana's hut before he heard her behind him.
"Ástríce!" She shouted, throwing him across the forest. The impact shook Merlin badly, but he didn't have time to nurse his injuries. If Morgana caught him, he'd be in a much worse situation than he was before. Merlin stumbled up as he ran, dodging Morgana's spells. "Fylwérig!" [stun] He saw a bolt of light pass next to him as he slowed time to leap behind a tree. It missed him by inches.
Merlin took off running again, turning as he ran to yell "Fréosan!" [Freeze] He combined the spell with his command of time to freeze Morgana in place, but the spell missed and she kept running behind him. He saw Morgana's dress and had sudden inspiration. "Becnyttan!" [Tie/Bind] He cried, tangling Morgana's dress beneath her feet and tripping her up. Merlin didn't risk looking back to check up on her.
Merlin slowed time to allow himself to run faster from her, not letting his exhaustion getting the better of him. He knew Camelot was only 2 or 3 hours walk away from here, making it easy for Agravaine to find her. He would guess that he was in the direction of the white moutons, deep within the forests of Camelot. He ran for as long as he could, but passed out near the thick forest outside Camelot. Captivity and magical exhaustion getting the better of him, he sat down and immediately exhaustion overcame him.
He was woken not long later by none other than Gwaine. "Merlin!" He shouted joyously. "It's good to see you mate." Merlin saw the other knights of the round table behind him. "It's Morgana," Merlin said, climbing to his feet. "I found her hideout - she took me captive, I escaped. We need to get back to Arthur, capture her before she gets too far. She'll be running now that I know where she is." Gwaine's face grew serious, as did the two other knights who were with him. "Lets go," He said, as he pulled Merlin onto his horse. They rode hard and were back in Camelot within the hour.
Merlin characteristically burst into the council room. "Sire!" he said, aware of the councilmen in the room. "Merlin!" Arthur exclaimed. "It's good to see you!" Merlin waved him off before he started in on the joyful reunion routine. "It was Morgana who captured me," Merlin said. "She took me to her hideout. I escaped, but if we don't act fast she'll abandon it. If we hope to capture her, we need to leave now."
Arthur's face hardened, and he turned to the council meeting. "I'm afraid this meeting is adjourned," Arthur said shortly. They nodded in understanding. He turned to the knights of the round table. "We ride immediately. How far is the journey?"
"Not two hours if we ride hard," Merlin said in response.
"Then we leave now."
It was less than two hours later that the group arrived at Morgana's hovel. "We're here," Merlin said. "It's just inside these doors." Arthur led the group inside and pushed it open, Merlin entering behind him. "We're too late!" he exclaimed. All of Morgana's potions, poultices, books, everything was gone. The hovel was abandoned.
"Well, it's at least worth taking a look around for clues," Arthur said. As soon as all the knights got inside the small hut, they heard Morgana's voice.
"Elbogan ond clústor!" [shut and lock] The door swung shut and locked behind them. As soon as the shock registered on their faces, they heard Morgana say "Befón eac beadumægen." [contain with force] Runes drawn around the edges of the hut lit up, and a blue shimmering wall covered the inside of the hut. Morgana then emerged from a cupboard in the hovel.
"What have you done!" Arthur exclaimed. Gwaine beat against the door behind him, but the force of impact never reached the wood. Even Percival's blows did nothing, his broad fists beating usefully against the force-shield. Merlin saw Elyan try and rub out the drawn runes on the floor, but knew erasing them would do no good. More likely than not, Morgana bound this spell to herself instead of leaving them in flimsy runes. How she had set up a complex spell so quickly he had no idea, but bet that it had something to do with being a priestess.
"You know very well what I've done and what I will do," Morgana said. "Thank you Merlin, this was better than I'd ever hoped for." She smiled sinisterly at Merlin's scowl. "Oh, not only for bringing him to me," Morgana said, "But for the chance to break his heart."
Merlin's face twisted in anger at Morgana, and hurt crossed his features as he turned to Arthur. "I'm so sorry, Arthur."
Arthur put his hand up, dismissing Merlin. "You did not intend this," he said shortly. "I know you are no traitor."
"Oh, don't be so sure," Morgana crowed. "Merlin has a secret you don't know." Merlin looked at Morgana with more betrayal than he'd ever felt in his life. "Oh don't look at me like that," she spat. "After all, you hid from me too."
Merlin's face twisted in pain. "And it breaks my heart that I had to, Morgana. I blame myself for what you've become." He then turned to Arthur, eyes watering and seriousness in his gaze.
"I want you to know that whatever happens, whatever has happened and whatever will, You Know Me. You've always known me." His tone was slow and grave. Behind him, Morgana sadistically enjoyed his pain, a smirk growing on her face. "You know me. I was always and still am the clumsy idiot that brought you your breakfast, and am still happy to be your servant until the day I die. I would gladly die for you. None of it was a lie." Merlin had a tear tracking down his face.
Gwaine spoke up before Arthur could. "I'll make sure he knows." He gave Merlin a meaningful look, and Merlin momentarily entertained the idea that maybe Gwaine knew his secret already. Arthur opened his mouth and motioned to speak, but Morgana silenced him.
"Béon forswigian," [Be silent] she incanted dismissively, and the next words out of Arthur's mouth came out with no sound.
"I will tell you what is going on, dear brother." The spat the last few words. "Your precious Merlin is a sorcerer." Arthur looked at Merlin with abject disbelief written all over his face.
"No," he said. "He can't be."
"Luckily, we have a way to prove it," Morgana said. "I invented a spell myself in years past - it forces whatever magical potential a person has to the surface." Morgana smiled wickedly and powerfully. So she wouldn't just let him show Arthur herself, she was going to force him to show just how powerful he was. Well, at least Merlin wouldn't be mistaken for being on her side now because, if he knew Morgana, it wouldn't be a pleasant experience.
Arthur held up a hand. "I thought people had to study magic to learn it, and that every sorcerer made that choice." Arthur narrowed his eyes in the direction of his sister.
Morgana laughed. "This is the legacy of Uther," she spat. "Magic is a gift. You are born with it, and if you do not learn to control it it will eat you away from the inside. If you have magic and do not learn sorcerery, you harm others or die yourself."
"That is not true," Arthur said. "People make the choice for magic themselves." Arthur's tongue curled around the word 'magic'.
"If that was so, why did Uther execute children?"
Arthur fell silent as Morgana laughed. "But I digress. Let us see if your servant has magic. Oh, and Merlin?" Morgana smiled wickedly.
"This will hurt. Åcumendlicnes mægen sé drýcræftas cwylla wiðer gewill!" [Bring with force the magical well against the will.]
Merlin felt his magical core ripping up against himself. He felt his eyes burn and felt the power of his magic swirl around him. He had never brought his magic to the surface before with such force, had never even tapped a tenth of the power. He felt as if his arms and legs were being torn off of his body, and fell to the ground clutching his head. He felt screams tear from his throat as he saw his golden magic swirl around him. In his pain his magic lashed out, breaking shelves and furniture everywhere. Merlin's hands fell to the ground as he was on all fours, his head fell to the ground as he curled in on himself, the pain of his core being ripped away from his control greater than anything he felt before. He was dimly aware of his magic swirling around him in a thick golden mist, and of Morgana's wicked voice in the background. He bit his screams down to listen.
"The evidence is before your own eyes," Morgana said. "And I must say, I'm surprised at the amount of magic he has. I've never seen anyone with this much before." Arthur didn't believe her.
"You're doing this to him, somehow. I won't let you set my best friend up."
"No," Merlin panted. He looked up and his golden eyes met Arthur's. "I only use it to protect Camelot," Merlin said. He screwed up his face in pain. "I only use my magic to protect yo-" Merlin's speech cut into a scream, as Morgana threw out her hand and yelled wildly
"Ådl!" [pain] "Why do you defend Arthur so! He would have you killed, all of Camelot would have you dead!"
As if to prove her wrong, Leon drew his sword and said "Stop what you are doing, Morgana! It's purpose has been served - stop hurting Merlin." Morgana pouted.
"But it's turning out to be so much fun, watching my brother squirm. Tell me Arthur, what will you do about this sorcerer?"
"Remember Arthur," said Gwaine warningly. "Remember what he said." The knights around him looked at Arthur, clearly pleading him to stop what was being inflicted on Merlin. It seemed evident all of them, even Elyan, still trusted Merlin - at the very least, they didn't want his pain to continue. Arthur's indecision was evident on his face - it was tearing his heart apart to see his best friend being put through this but at the same time, every fibre of upbringing told him he shouldn't care now that he knew he was a sorcerer. Merlin raised his head again, tears streaming from his eyes and his cries of pain forming words meeting Arthur's gaze.
"Please… Arthur…" Merlin begged.
That was all it took for Arthur to become the deserving King of Camelot. His heart told him Merlin had done nothing wrong and that he didn't deserve this pain, and if Gwen and Camelot (And Merlin) had taught him anything, it's that his heart wouldn't lead him astray.
"Stop doing this to him, Morgana. Nobody deserves pain such as this." Arthur's voice spoke with a kingly commanding tone. Morgana cried as she broke the spell and Merlin crumpled to the floor.
"Why would you spare him and not me!" she cried. "He is just as much a sorcerer as I am!"
Arthur began defending his decision in spite of himself. "Merlin has not murdered in cold blood like you, Morgana. Merlin has never thrown any rightful ruler off of their throne, God knows why if he's a sorcerer. Merlin has never betrayed secrets of Camelot like you have." Arthur found himself believing his own words the more he went on. "If Merlin has magic he has certainly never used it for evil like you."
Morgana flung her arm out, but Merlin reached out his just in time and cried "Åsælan híe forhíe!" [Bind movement for them] She stopped moving and seemed unable to move or speak, although she was twitching and her eyes were flashing gold dangerously. Merlin quickly but shakily sat up, and cried quickly "Líesing fram befónir beadumægen!" [release from containment of force] The shield in the hovel disappeared and the runes darkened.
"Run," Merlin said shakily. Gwaine went to help him up, but Merlin brushed him off. "Morgana's going to break out of my spell any second-"
Just then, Morgana stumbled with a flash of her eyes, free, and cried "Forþ fleoge!"
The knights flew back but Merlin cried "Åþwænan!" [soften] before they hit the wall. When they hit the wall, the stumbled and were thankfully still conscious. Merlin wasted no time in flinging both of his arms out and yelling "Bealucræft bordrand!" [magic shield] A magical shield was erected in between Morgana and them, her spells hitting the shield and dissipating upon impact with the shimmering energy. However, Merlin was quickly weakening from the effort of the spell.
"Go back to Camelot!" He said with urgency to the knights. "I can deal with Morgana for now. Go, tell them she got away."
"What of you?" Gwaine asked hurriedly.
"I'll be fine," Merlin said, waving his hand. "Now go!" They saw the shield fracture for a second, and needed no more encouragement to file out the door.
Arthur was the last to leave, and even in his old age he never forgot the look Merlin gave him as he passed through the door. His eyes looked back, and despite the gold in his irises his eyes shined with a loyal sincerity, and his mouth turned upwards into his old, encouraging smile. His eyebrows pushed together as his face took on it's reassuring countenance, everything in Merlin's stance telling him that everything would turn out fine. It certainly would if Arthur had any say in it.