Note: A short idea I had, featuring a dark Merlin. This is an AU taking place after the first few episodes of Series 3, and thus there are some spoilers for that.
Nightmare after nightmare plagued Merlin, and he had not slept well for weeks. In every single one of them, Morgana ended up killing Uther, Arthur or both. He had no idea whether they were predictions of what was to come or not, but either way he had to take the initiative before it was too late.
You've done terrible things before, he thought to himself, but it keeps getting easier. Everything you've done was for Arthur, and if anyone gets in his way they must be destroyed. The Dragon was right, he's always right.
As he neared her room, he trembled so much that he had to stop for a moment and breathe heavily. Luckily, nobody was around to see his out-of-character behavior, or for what he was about to do.
There's no other way.
His face hardened, and he became deathly calm, fully resolved in his decision.
Merlin forced her door open, and before she could react, closed and sealed it with magic behind him. Morgana clearly saw this, momentarily stunned.
"You! You... you have magic!"
He didn't answer, but stood still and stared at her as her face filled with rage, and her eyes flashed gold.
"All this time I thought you were just a weak, cowardly servant. But no. I see now how you must have escaped what Morgause and I thought was certain death. You were very foolish in coming here, Merlin. Now I will tell Uther, Arthur and everyone else just who you are and what you've done to me. You will be burning outside my window before the sun sets."
"You are not going anywhere, Morgana." Merlin said, still calm. "You are absolutely nothing compared to me, and I will make sure Morgause also meets her end later as well."
"Do you think you can hold me with a simple locking spell?" Morgana said, smirking as she stepped forward to him. "And don't you dare threaten Morgause. She has been more of a friend and taught me much more than you ever could! You have been lying to me all this time, Merlin! Why did you send me off to the Druids for help and didn't tell me what you really were? Why?"
Her voice broke with her last words, and Merlin couldn't resist feeling regret. Everything could have been different if he had, if he had defied destiny.
"I wanted to! You don't know how much it hurt me to see what you were going through, but I couldn't! You are not more important than all of Camelot, and Arthur!"
"Arthur, Arthur, Arthur! It always comes down to him, doesn't it? You poisoned me to save Arthur, and thought absolutely nothing of me!" said Morgana.
"He is going to be King. You are going to be nothing more than a hateful, spiteful witch. Or, at least, that is who you would have been, but I can't allow it. I might as well tell you why I came here, Morgana."
"Why then? Did you realize that you are on the wrong side and wish to join us? I just may be able to forgive you, but most likely I will still go to Uther unless you can convince me otherwise!" she said, the spiteful smirk returning.
"Enough!" yelled Merlin, with a magical force that propelled her backwards into the wall. "And don't do that stupid smirk again! Can you be more blatantly obvious that you're planning to kill Uther or raise an army of undead or some other evil? I don't know how anybody else hasn't noticed, but I have! You don't know how difficult it has been for me for over a year! A year, Morgana! There was no other way to undo the spell besides ending your life, and I didn't even know you were alive until you came back to Camelot! And then you told me that you understood, and I believed you! I thought that you could be changed, that you could swerve off the path that destiny made for you, but I was wrong."
Morgana tried to scream, or indeed even move, but she could not. She was completely paralyzed and helpless against Merlin's magic, which was far more powerful than her own. There was no escape.
"I keep dreaming of you killing Uther or Arthur, and I know that as long as you are here, it may come true. It took me quite some time and a lot of sneaking around to find the proper spells, but I now have them. I'm so sorry, Morgana."
Merlin then touched his index finger to her heart, and after a short incantation, Morgana crumpled into a heap upon the floor. She was dead.
The next hour or so was a complex exercise in some of the most darkest of magic, necromancy. The plan was to have Morgana, or at least her body, live in Camelot as she usually did. She would be an empty shell, completely under his control, soulless, and powered by a constant drain on Merlin's magic. She would walk, talk, eat and sleep as if nothing was out of the ordinary.
Merlin hoped to never have to use that kind of magic ever again. It was wrong.
Morgana was too strong to be mind controlled for long and he couldn't simply have just killed her, so the plan was to control her undead body, have her reveal her magic, and be executed. Nothing would trace back to him.
She would be freezing cold to the touch. A simple way around this would be to re-apply a warming spell every few hours, but he would risk getting caught using magic. No, he would have to enchant Gwen and Uther (the two people in Camelot likely to touch her on a regular basis, especially her maidservant) to think that she had bodily warmth. There was also no way that he knew of to make Morgana's body to breathe or for her heart to beat, so he would also have to put spells on them both for these reasons also. Against everyone else, he hoped nobody would notice.
He couldn't keep up the charade for long anyway, since it was very taxing to maintain dozens of spells, especially for long lengths of time. What he used would also only halt the decomposition for a time before she would begin to rot, and everyone would notice that for sure. No, within a few days he would have Morgana attack Uther with magic (non-fatally, of course) in front of the court, and her threat would be over.
As he was in the throne room with Arthur, who was talking with his father about knight training or whatnot (he really wasn't paying any attention), he glanced at Morgana on her throne. He mentally commanded her to flash a quick smile at him, so much different from the disgusting sneers he had grown accustomed to. For a moment, she looked like the old Morgana, before everything bad had happened. He would always remember her.
Maybe everything could have been different, I'm sorry. I'm so sorry, Morgana.
She rose without a word from her seat and turned toward Uther, interrupting Arthur as she held her hand out, eyes burning gold.
It keeps getting easier.