Board Thread:Rewatching "Merlin" - Season One/@comment-5102537-20130302204954/@comment-5674726-20130306000854

Nimueh may not have known of Arthur and Merlin's supposed destiny. Her fellow priestesses, Morgause and Morgana were unaware of the existence of the prophecy. Perhaps the Triple Goddess knew that it was just one of many ways that things could play out and didn't want her priestesses to put their faith in an 18,569 to 1 chance that Arthur and Merlin would manage to create a golden age for Albion.