Nauthveli

Nauthveli is a sea-monster from Icelandic folklore and one of the Illhveli ("Evil Whales") which acted as major antagonists in many stories across the region.

Role in Folklore
The Nauthveli, like all creatures of folklore, could vary depending on the story but tended to be seen as an abnormally large, aggressive whale - normally bicolored like a cow and having a large bovine-esque head, much like a bull, from whence the monster earned its alternative name of "Ox-Whale".

Nauthveli was infamous for its terrifying bellow, which it would make whenever it was hungry (which was often) - the bellows were supernatural in nature and could cover huge distances, causing the ground to shake and oars to fly out of sailors' hands - whenever the Nauthveli's bellow was heard it was forbidden to set sail into the ocean, likely as a means to try and prevent deaths from the rampaging beast.

While Nauthveli shared the "Evil Whales" joy of murdering sailors and wrecking boats it was more famous for its extreme love of beef and its bellow was hypnotic to cattle, making them run off cliffs into the ocean, where the whale would proceed to toy with the animals before biting them in half and devouring them.

If any boats had cattle onboard this would also attract Nauthveli; once Nauthveli harassed a vessel off Grimsey until they released the one cow on board, who promptly dove into the sea.

Cattle had to be locked up for days until the Nauthveli’s spell wore off, and indeed cow-herding was strongly discouraged in areas where the Nauthveli had been heard. Sacrificing one bull or cow usually satisfied the Nauthveli, making it safe to go out to sea again.

Abilities
Nauthveli was supernaturally large and strong, capable of sinking ships with ease - it had a distinct bellow that had both destructive capabilities such as inducing earth-tremors, knocking oars out of sailors' hands, churning the seas and hypnotic properties in regards to cattle.