"Do you really wish to know?" — Spoilers from the books and/or adaptations to follow!
Agloval was the proud and arrogant duke of the seaside town of Bremervoord, who fell hopelessly in love with the mermaid, Sh'eenaz. There was one rather obvious obstacle to this love story and the duke hired Geralt to overcome it. When the witcher failed to convince the mermaid to give up her life in the ocean and join Agloval on land, the duke decided to pay him instead for investigating the mysterious deaths of several local pearl divers.
This led to the discovery of a mysterious race of underwater dwellers. A war was narrowly averted between the people of the sea and their land-based counterparts when Sh'eenaz decided to finally to give up her tail and walked to join her lover in marriage.