A Monster Calls {Sam Winchester}

Since the death of Arthur Pendragon, his life and death had passed into legend, forgotten by a world of rationalism and political parties. But there are still people waiting for him to return.
Many anticipate the return of the king, among them Rhiannon Griffis.

When the Men of Letters spread their network through Europe, ridding the continent of magic, Rhiannon and the others remaining from the time of Camelot are forced to flee to America, reliving terrible memories of the Great Purge of King Uther. Despite hopes that life could be better in America, their practice of magic is still severely hampered - now by the American hunters, who often shoot first and ask questions later.

But when news of the apocalypse reaches their little community, the possibility of Arthur's return is suddenly very real. Unfortunately, there are still a few problems... One, getting back into Europe. Two, the apocalypse is still happening. And three. Rhiannon may or may not have gotten on the radar of every hunter in the country...

And by someone's almighty design, the Winchester brothers found her first.

This is a bit of a Merlin crossover, ergo I do not own Merlin or Supernatural. But all original plot and characters are mine, and God help you if you steal them.