Undine

A knight marries a foundling raised by fisherfolk, and learns on the wedding night that she is a water spirit who has gained a soul by loving him — and that her uncle the stream will avenge any wrong done to her. Fouqué published "Undine" in 1811; Heine praised it, and every later mermaid story, Andersen's included, is written against it. The soul is the whole problem: with one she can suffer, and does.