I'm sure that you don't know who is Aengus?
He is an irish divinity, one day he fell in love with a pretty girl, Ethal Annubal's daughter, another immortal, when Aengus asked her father for her hand, she learned that she is most powerfull than him and that she lived a year as a swan and a year as a human. If Aengus could recognized the swan among other, he would married her. He achieved it and could live with his love.