Hrosshvalur

Hrossavlur 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 Hrosshvalur, or “Horse-whale”, is among the most cruel and dreaded of the Icelandic illhveli, or “Evil Whales”, with only the Stökkull and Raudkembingur rivalling it in malice - as such it is unsuitable for eating and consumption of its flesh was outlawed with severe punishments for any who did so.

It loved to engage in acts of violence and was especialy prone to jumping onto ships so as to destroy them with its size, which could vary from 15 to 40 meters in length - the creature's large eyes are its main weakness, which can be used to defeat the monster if a victim is quick and smart enough.

When not sinking ships this monster would make huge waves via whipping its tail and call forth bad weather to further torment seafarers - it also emitted a terrible smell and was in general a dangerous opponent, just like the other "Evil Whales".

In the 13th century, Hrosshvalur surfaced alongside a ship but was promptly bombarded with every heavy implement available, which caused it to sink back below the waves before it could cause further damage - in another  tale Hrosshvalur attacked the heroes Hjalmper and Olvir; it was defeated with the help of a Skeljungur (“shell whale”), vagnhvalur (“chariot whale”, or killer whale), and two vultures. A cutlass thrown into one of its large eyes weakened it significantly, and it was seemingly torn apart by the whales.

Hrosshvalur is associated with the dark arts, since its size and ferocity made it an excellent accomplice for sorcerers and witches bent on destruction, and a perfect form to assume when causing chaos.

In the Kormaks saga, the witch Dorveig transforms herself into an image of Hrosshvalur to attack the brothers Kormakr and Dorgils. They recognize her from her eyes, and drive her off by throwing a javelin into her back.

Abilities
Hrosshvalus is supernaturally large and strong with greater intelligence than a normal whale, since it is said to delight in chaos and may even ally itself with evil-minded magic users : it can emit a terrible smell and manipulate the weather, often to create storms or other hazards to those who travel the sea.