This is an AU story so I have taken a few liberties with the show.
Uther has no trouble with magic, nor has he killed or outlawed magic. He simply does not believe in the supernatural. He is not the king nor is he Arthur and Morgana's father. He was the late king closest advisor and the kingdom was trusted to him until Arthur is of age to rule
Numueh, Morgana and Morgause are allies. Their goal is to rule over all Britain, not just Camelot. Their rivals are Arthur and Merlin. Morgana, Morgause and Arthur are brothers, not half brothers.
Altought a not a crossover story, this has common elements with the tv show dexter.
Now to the story.
"18 years is a long time" Uther thought as he watched Arthur being crowned. As ruler he could not allow his nature to roam freely; he had used all his will to control the dark passanger, but the waiting was over. He had already begun to investigate a famous killer in another kingdom.
Although he possessed no human emotions, he was a master as passing as one of them. He raised his cup and gave toast for the king. An act that was required of him.
He thought strange that in the rigid society he lived in, how he, the bastard son of a farmer, had escalated his position into being one of the king's closest advisors.
Perhaps it was fate that a knight found him covered in the blood of his mother, as she was killed and tortured by bandits. The knight raised him as his own son, but Uther remembered everything.
The knight knew there was something wrong with him and had taught him "the code". It had helped him avoiding being caught as he punished those that harmed the society.
On the case he was working now, all the bodies have been found without blood. Peasants spoke their usual superstitions, but he paid them no attention.
Meanwhile.
"Are you sure this is what you want?" Nimueh asked Morgana.
Morgana stared at the sorcerer, who had returned "changed" from her travels to Eastern Europe.
"I am sure of it"
Nimueh smiled sadly. "Morgana, this is more a curse than a blessing…this sort of "magic", is known in those lands as the dark gift. It's a magic darker than any I had ever seen here in Britain…It was not my choice, but by force that I received. It may have increased my magical powers, but the prize is just to much"
Morgana answered
"Britain deserves a better ruler than Arthur, and I am willing to be their queen for all eternity…with my powers increasing, no one will be able to stop me"
Nimueh insisted
"Morgana, I also wish to see you on the throne, but at what price? I will be condemning you to eternal thirst. A creature of the night, neither alive nor dead…also, there was another who fallowed me from those lands. I have felt his presence, and unlike me, he has no qualms to drain their victims…I am afraid you would not be able to control yourself"
Morgana said firmly
"This is my choice, please don't deny me this"
Nimueh seemed doubtfull for a moment, but then Morgana saw as Nimueh eyes glowed and her fangs extended.