Merrow no Majinai

"My sister didn't just vanish into sea foam." The protagonist is a mermaid who was stripped of her beautiful tail—transformed into grotesque octopus tentacles—as punishment for breaking a sacred law. Living quietly as an outcast, she watches as her youngest sister falls in love with a human prince. Her sister makes a pact with a sorcerer, gains a pair of legs, and ascends to the land. Yet, what awaited her was not a happy ending, but a sudden disappearance. "I have to go save her." The protagonist, too, acquires legs and infiltrates the castle in human form. What is truly happening inside the castle where the prince awaits? And what has become of her missing sister? [Translated from Novelgame]