What if Nimue was the first Dark One and this is the second (“Vortigan”)?
Maybe Merlin tried to get Nimue to let go of the darkness, but before that could happen, Vortigan killed her and became the next Dark One. And maybe Vortigan was Merlin’s friend or something which would explain why he couldn’t kill him. I mean, if this is Nimue, then it’s gonna feel a bit odd if she was just killed and succeeded by a random DO we never hear about.
But it probably is Nimue cause that’s the kind of thing they would do.