Nuckelavee

The Nuckelavee is a monstrous sea-demon that was commanded in centuries past by Fisherman to lay waste to the mystical undersea kingdom of Hildaland only to be defeated by the song of the Seal Maiden.

However many centuries later Fisherman would succeed in reviving the Nuckelavee and used it to enforce his will upon Hildaland once again, prompting the events of the "Message In A Bottle" storyarc.

Note: The Nuckelavee is a very real creature of Orkney folklore - the monster that appears in my stories is a homage, for more information on the myth itself feel free to search the internet (it is quite fascinating, at least to me)