This is just one version of this German mermaid transformation story, there are many others.
In this version, Lorelei is a beautiful young maiden, too beautiful for her own good. All of the local young men fell in love with her and she could not resist their advances. This brought scandal and shame to her small town.
Despite her many lovers Lorelei has one true love. This one man however rejected her.
Pining for her lost love and feeling guilty for all disgrace she has caused, Lorelei longs to end her own life. The bishop brings her to trial for her wanton ways but is seduced by her beauty and unable to sentence her to death. Instead, he arranges for three knights to take her to a nunnery.
As they passed a cliff above the Rhine Lorelei asked for permission to view the river. The knights assented. While gazing at the Rhine Lorelei suddenly said, "The boatman down there is my lover!" and jumped over the edge.
Death transformed Lorelei into a siren. Since then, many boatmen have been lured onto the dangerous rocks by her beautiful singing.
Mermaid Transformation: Lorelei --> Mermaid Transformation Stories