I kind of went for a scylla/Ursula thing here.

And in the midst of the crime scene Vendetta’s bracelet was found. Now of course she didnt do it, she was framed. But the king didnt want to hear it. Demanding she was brought to him and without leaving her time to explain he cursed her. Banished from atlantis and now a horrifying creature.

Sure! So Vendetta lived in atlantis, she used to be a beautiful mermaid. Being childhood bestfriends with the king along with his person herbalist. But one day someone had poisoned the food that was prepared for the king (the chef was found dead).