Ylerys
From The Griffin's Crier
The Silver Discs of Elaran detail the battle between an aquatic elf tribe and the odd half-human, half-fish followers of Ylerys long ago. They defeated the cult, and slew the demon, sealing its body in five urns. The urns were buried in the undersea ruins of the demon's temple.
The Discs close with a warning that the seals will not last for all time, and that a handful of cultists fled during the final battle.
Ylerys is know as the herald of Dagon, an ancient proto-demon who existed before demons, and even before gods.