Contrary to popular belief, Sirens and Merfolk are the same species. The confusion is understandable: Sirens being of a more bird like appearance while merpeople obviously favored the sea. A peculiar transformation takes place on the night of the new moon.
The sea must be perfectly still, not even a ripple from the winds can be present. The stars all must shine brilliantly, none of their light overshadowed by the moon. Only under these conditions may the hopeful merperson make the transformation.
Some say the transformation is painful. The bones break and reform themselves. Scales recede and feathers grow in a matter of seconds. Time itself stills for them as the knowledge of flight and human song is given to them and the language of the sea stripped away. It is an eternity of agony and regret for some.
For others, it is their true birth.