Frostbacks

Leviathans that prowl the Sea of Black Ice, Frostbacks are akin to gargantuan turtles, protected from the cold by layers of blubber, and encased in hard armoured shell that they use to crack through the icefloes to hunt the unwary upon the surface.
They swim slowly through the depth, powered by their huge flippers, and will sense their prey upon the suface of the ice above. When ready, they will swim mightily above, and smash through the ice. Their mouth is ringed by long tentacles hich they use to slide through the ice cracks and drag the unfortunates to their watery doom.
They get their name, 'Frostbacks' from the hardened ice that accumulate upon their shells. They have been known to sleep upon the surface of the Sea of Black Ice, with heds and limbs retracted into their shell for protection. At these times they look like just another iceberg upon the waters. Many a ship of The Icerunners Guild has discovered their mistake too late.