The Little Mermaid
In Hans Christian Andersen’s version of the tale, Ariel makes a deal with the sea witch in order to become a human and pursue the prince she saved in a shipwreck. She gets a pair of legs in exchange for her tongue.
However, part of the deal is that every step she takes with this new pair of legs will feel like walking on shattered glass. If Ariel gets her true love’s kiss, she can remain a human, but if she fails, she will die.
The prince never actually saw Ariel’s face when she saved him, so he ends up marrying someone else. The sea witch tells Ariel she can just turn back into a mermaid instead of dying if she kills the prince.
Of course, Ariel cannot bring herself to do that. Instead, in dark Disney fashion, she throws herself into the ocean and dissolves into sea foam.