This idea came to me a while back and I decided I may as well write a little oneshot about it. Just to get it out of my system.

This takes place just after season 4.

It was the only form of magic Uther could not condemn or even destroy. He would have had more luck trying to stop the rivers flowing or trees growing really. Nobody knew when it had first started but it went back centauries in the history books.

The Soul Words.

The Soul Words referred to the name that appeared permanently tattooed on your skin once you met your Soul Mate, hence the name. Arthur had been forced to wear long sleeves ever since he first kissed Gwen before the tournament years ago because afterwards he was shocked to find the word "Guinevere" written across the skin in a ring around his arm just below the shoulder in blue.

There was no guarantee where your words would appear, only that the writing style and place would be exactly the same on your partner. So naturally Gwen had blushed when she showed him his own name written around her arm also, in the exact same shade of blue. Once when he was younger Arthur had seen a man who was unlucky enough to have the name of his beloved tattooed across his forehead.

He remembered his father had "Ygraine" written across his palm in black. When he was younger he'd thought that was just the colour the fate had chosen for him, it was only later he learnt the significance. Gaius had told him when he was around eight what black words meant.

Death.

The name was only ever black if its owner was dead, before Uther's Soul Word had been red.

Now that he was king and finally able to marry Gwen he'd told the story about how they'd kept it a secret for years. The people had loved it, called them Camelot's Star Crossed Lovers. Gwen had blushed at that.

Arthur had never given any thought to Merlin, he'd never gotten close to anybody and as far as Arthur knew he was still Wordless. Once people gained their Soul Word it was generally a topic of conversation, at least with close friends. Which is why it's discovery had been so shocking.

The knights were in the weapons room, packing away their things after training. They were all chatting happily amongst themselves, Elyan had been in an especially good mood the past week since his Word had appeared. Percival teased his relentlessly about wearing a bracelet.

"Just because it is around my wrist does not make it jewellery!" Elyan argued, but he was grinning.

"Just keep telling yourself that." Gwaine jibbed.

Merlin was carrying in Arthur's cross bows at the time, groaning when he noticed the stool was missing and he'd have to stretch to reach the hooks where they belonged.

"Very funny guys." Merlin smirked, not really minding; he knew one of them had hidden it.

"We thought so," Leon grinned.

So Merlin stretched upwards to reach the furthest hook and place the corresponding crossbow upon it when his tunic rode up. Revealing a small patch of skin on the young mans back.

All the knights and their king were silent in shock. Because on the small of Merlin's back just above his right hip was a Soul Word.

"Freya"

Merlin had never mentioned meeting a girl before but what made the group share looks was not the name, but the fact that the words were pitch black. Who ever this had been, she was dead.

…

"Anybody have any idea when that happened?" Leon asked after Merlin had left, not realising the other men had seen his Word.

"I remember back, about a year after he arrived he was put in the stocks in the middle of summer as punishment by my father." Arthur said aloud.

It was a cruel practice to lock men in the stocks shirtless in summer with their backs to the sun so that it would burn badly through the day. Arthur had secretly let Merlin out early and given him the evening off.

"It wasn't there then."

"So this happened while we knew him?" Percival confirmed.

"Must of." Elyan nodded, "How could we not know?"

"Most people are ecstatic when they meet their Soul Mate and go into deep depression when they die." Percival added, "But he's never shown any of those signs."

"Maybe we saw wrong." Arthur shook his head, "There is just no way we wouldn't know. You know Merlin he can't keep a secret to save his life! There is just no way he wouldn't tell me."

"I've never seen anybody so young with black words." Gwaine grimaced.

…

The warlock was getting suspicious, for the past few days the knights had been acting strangely. They were jumpy and much nicer than usual. Not that they were mean to him but, they were always doing him favours and asking him questions about his life.

"Where did you grow up?"

"Why did you come to Camelot?"

"Met anybody special since you were here?"

"Any idea when you'll get your Word?"

The last few had hurt, but he'd feigned ignorance at them. Even Arthur was giving him less chores and Gwen had bought him extra food from the kitchens. Royal standard food too, fresh fruit, garnished meats and even a cut of venison. Something was going on…

…

They spent a full week trying to man up and just ask Merlin about this Freya person. Bringing up somebody's dead Soul Mate was a touchy subject, this time even more so since they were not supposed to know about them.

It was Percival who finally took the plunge. They were sitting around the fire while out on a hunt, they had just finished eating and Merlin had just sat down after finishing his chores for the night.

"Merlin, who's Freya?" The gentle giant asked.

Merlin was so shocked he choked on the water he'd been drinking.

"What?" He coughed, "How'd you-?"

"We saw your Word, the other day in the armoury." Gwaine smiled weakly, "Been trying to ask you about it ever since."

"Wait, all of you?" Merlin sounded nervous; Arthur noticed his eyes flick to the king. So he was afraid of Arthur knowing more than everybody else? Why?

"Yes." Leon nodded, "So, who was she? You obviously met her after you moved to Camelot."

The servant gripped the water skin tightly.

"She was just a girl, alright?" He muttered before getting up, "I'm going to get more firewood."

And with that he practically flew off into the trees and away from the group.

"Well, that went well." Gwaine nodded.

"Maybe we should just let him be?" Elyan shrugged, "He obviously doesn't want to talk about her."

"Even so we can't just let him wander around in the forest at night." Arthur replied getting to his feet, "You know how clumsy he is, he'll fall down a ditch or into a river."

"I'll come." Gwaine offered, "Lets go find us a Merlin!"

…

Arthur felt bad about ambushing his friend like that, even more so when Gwaine and the king finally found him. The servant was angrily throwing stones at a tree with a look of malice on his face.

"Maybe I should talk to him." Gwaine offered.

Arthur nodded, despite his quite close friendship with Merlin over the last week he realised that the man never really trusted him with his private life. There was always going to be that barrier, even if was thin. Gwaine was relaxed and accepting of just about everybody, Merlin would be more likely to open up to him. However instead of going back to the camp, the king stayed behind the tree out of sight to the two men.

"Sorry we upset you mate." Gwaine said quietly.

"It's fine." Merlin sighed looking more dejected than Arthur had ever seen him, "It's natural you'd be curious."

"So, what happened?" Gwaine asked gently sitting down next to the servant.

"She was killed." Merlin said simply throwing another rock. This time there was no anger; it was like it had all drained away leaving a heavy feeling of loss and pain in the air.

"How?" Gwaine probed further, Merlin looked away.

"It wasn't his fault okay?" Merlin replied much to Arthur shock, he?

"If I tell you, you have to promise not to tell Arthur." Merlin turned back to Gwaine.

The king couldn't help but feel hurt at that; Merlin didn't want him knowing personally?

"I guess, but why?" Gwaine asked, "Why can't Arthur know?"

Merlin sighed.

"He didn't know, it's just as much my fault."

"You're not making sense." Gwaine replied.

"Arthur killed Freya." Merlin said finally, the king almost fell over.

"What?" Gwaine breathed, Arthur could see the sympathy on the usually happy mans face.

"He thought she was a threat." Merlin explained closing his eyes and holding in tears, "I tried to get to her in time but by the time I got there he'd already dealt her a mortal blow. So I snuck her away a-and bought her t-to the lake outside the city, she loved the water. So I stayed with her until she died in my arms and gave her a funeral."

"All by yourself?" Gwaine gaped, Merlin nodded unable to keep speaking.

"Gods Merlin why didn't you tell anybody?" Gwaine continued, "How long ago was this?"

"Around three years." Merlin half sobbed.

Arthur felt like the guilt was going to kill him. Three years Merlin had hidden this from him. Three years of loyalty and friendship after he'd killed his Soul Mate. He hadn't even held a grudge; even now he didn't blame Arthur even though there was no one else to blame.

How could he of done that? If Merlin was somehow responsible for Gwen's death Arthur doubted he could ever forgive him, let alone just go on without a noticeable fissure in the friendship.

"You can't tell Arthur!" Merlin repeated, "He'll never forgive himself."

He was right.

"But, didn't you tell anybody when the words appeared? Surely Arthur would of spared her if he'd known?" Gwaine spoke up, covering his mouth as soon as the words had left it, "Sorry I didn't mean-"

"It's alright." Merlin smiled sadly, "I couldn't tell anybody, I'd rescued her from a bounty hunters cage, the whole city was looking for her. I hid her in the catacombs, bought her food and stuff."

That jogged something in Arthur's memory, years ago, he was sure Merlin was stealing food from him. Something that was so unlike the honest man he knew, he'd even caught him smuggling chicken in a pot after saving him from Halig the bounty hunter.

Bounty hunter. Didn't Merlin just say he'd rescued this Freya girl from a bounty hunter?

"She was cursed." Merlin's voice wavered, "By a witch, she transformed into a monster every night and couldn't control herself again until after she'd killed somebody. 'Cursed to kill forever more'."

"And Arthur killed her when she was this monster." Gwaine surmised, "I'm sorry Merlin."

"She wouldn't hurt me, when I was around her she could control herself despite the curse." Merlin was practically sobbing now, "I was going to take her away from Camelot, so it wouldn't matter, I could look after her and she wouldn't kill anymore."

Oh God Arthur remembered that girl, the one with the tattered red dress and dark hair who turned into that winged cat. He had dealt it a mortal blow; he had always wondered why he never found the body on the city outskirts. Now he knew.

"But I made a mistake, I left her and she changed while I was gone…"

"And because you weren't there she went out into the city and Arthur found her." Gwaine surmised.

"She didn't want to take me away from my life in Camelot, she tried to get away before I came back but the bounty hunter found her and she couldn't get out of the city in time and she changed." Merlin corrected, "We only knew each other for a few days."

Arthur had never seen Merlin cry so much. It was like the time with the dragon lord only so many times worse. How had he kept all this to himself this whole time? Arthur was the worst friend in the entire kingdom.

Gwaine stayed with a hand on the younger mans shoulder while he cried, Arthur returned to the camp unable to watch anymore. When he got back the others knew better than to ask what had happened, his face was ashen. Nobody questioned Merlin when he came back without any firewood and went straight to his sleeping roll, he was asleep in minutes.

"I heard." Arthur said simply, Gwaine sighed and relayed the story to the other men, Leon swallowed guiltily, he'd been there that night as well. Nobody felt worse than Arthur though.

"He doesn't blame you." Gwaine reminded him as they went to bed.

"Doesn't mean I'm not to blame." Arthur replied.

I know I made a few subtle changes to the Lady of the Lake episode, like Merlin knowing about the curse before she changed but other than that it all happened like in the episode.

I plan this as a one shot but if people really want it I might write another chapter about Arthur confronting Merlin or something.