Mefolk

Merfolk were once humans that lived on an island. They all worshiped the god of the sea – singing songs on how great he was and the how awesome the oceans are and gave great respect to the ocean waters. One day, a group of mages ransacked the town and sank it into the sea. To help save the people that loved him so, the sea god turned the people into merfolk.
The merfolk give offerings to the god of sea in the form of blood – to show that everything is made of water.