See caption
Fishing boats in a storm on the surging sea, by Joseph Mallord William Turner, depicts the kind of sea conditions in which the marool might appear.
Wikimedia Commons

The marool is one of the most malevolent of the monsters in the waters around the Scottish Shetland Islands.[1] It was described by the folklorist Jessie SaxbyAuthor and folklorist from Unst, one of the Shetland Islands of Scotland. She also had political interests and was a suffragette. in her Shetland Traditional Lore (1932) as a sea-devil in the form of a large fish, which appears particularly during storms when the sea foam is phosphorescent,[2] known as mareel:[3]

He had a crest of flickering flame, and eyes all over his head … He delighted in storm, and was heard to shout his wild exultant song when some luckless bark went under.[3]