Once the little mermaid got her legs she found the prince. Over time her and the prince became good friends but nothing more. When it was time for the prince to marry he took the temple woman's hand thinking she was the one who saved him that day.
Will you marry me?
On the day of the wedding the little mermaid was there knowing her end was near. Later that day her sister's came up from the surface and told the little mermaid she had to kill the prince in order to become a mermaid again. They then gave her a magic knife.
We miss you
Please come back to us
You have to kill him
The little mermaid went to go and kill the prince but she couldn't do it. She then threw herself into the ocean making her turn into form on the shore but because of her good deed after 300 years she can rise up to were no mermaid has gone before.