« Une once d'abnégation » est une des nouvelles dans la collection L'Épée de la Providence d'Andrzej Sapkowski. Elle raconte l'histoire d'un couple qui viennent de deux différents espèces: une sirène, Sh'eenaz, et son prince humain au nom de Agloval.
Agloval is the proud and arrogant prince of the seaside region of Bremervoord who is hopelessly in love with the sirène, Sh'eenaz. There is one rather obvious obstacle to this love story and the prince hires Geralt to overcome it assuming that since the witcher can speak the Langage ancien, he can overcome any negotiating issues. When, despite his command of the language, the witcher fails to convince the mermaid to give up her life in the ocean and join Agloval on land, the prince decides that the contract was not fulfilled. However, just to ensure that the money not go to waste, he decides that it should pay instead for witcher investigating the mysterious deaths of a number of local pearl divers.
This leads to the discovery of a mysterious race of underwater dwellers. A war is narrowly averted between the people of the sea and their land-based counterparts when Sh'eenaz mysteriously intervenes and stops the conflict. Ultimately, she decides to give up her tail and literally walks to join her lover in marriage.