Part of a series I am working on.
In Irish mythology, Caer Ibormeith was a daughter of a Prince of the Tuatha De Dannan.
Every alternate Samhain she would change into a swan, in which form she would remain for a year before becoming human again the following Samhain.
She eventually married Aengus of the Tuatha de Dannan, but first he had to pick her out, in swan form, from a group of one hundred and fifty other swans.
Having chosen correctly, he turned into a swan himself and they flew away, singing beautiful music that put all its listeners asleep for three days and nights