Once every thousand years upon a night as silent as the others a blue moon will appear. Only upon the night of the blue moon do Golden tail mermaids lose the ability to hide their identities. Making them vulnerable to attack. Can Merlin keep his identity, and powers hidden among the crowd, or will Arthur finally learn the truth about his magical merservant.
(I don't own merlin, or the charcters. This is merely a fanfiction.)