The Illvveli, which in Icelandic means Evil Whales, are a very special group pf sea monsters with a set of common attributes:
1. They all play important roles in Icelandic Folklore, and many of them appear in Viking Sagas alongside other very important monsters and mythical beings (not to mention gods and goddesses).
2. They are all inedible. First, because they are extremely hard to hunt and kill, but also because their meat is at times poisonous, or it may disappear as it is being cook, and because eating their meat may result in terrible curses or the evil transmutation of those who have eaten it. In fact, for long time it was a punishable crime to eat them, so even if you survive the experience, you may spend some time in jail and the rest of your life ostracized by your peers, who fearing the possible curses coming, will stay as far away of you as possible.
3. They are evil by nature, for they delight in the destruction of boats and ships, and in killing humans and other creatures, like benign whales and smaller sea mammals.
Nowadays, with the exception of a few ancient specimens, Illhveli are believed to be extinct. But, if you are unlucky enough to run into an evil whale, try making full speed towards the sun for ots blinding rays may dazzle them. If you encounter them at night, land is your best chance to survive, but only if you're fast enough to out-swim the beasts.
Fortunately, a few of these monster whales have particular weaknesses that can be exploited in times of need. The Raudkembingur (Red Crested Whale) will die of frustration if you manage to outrace it and escape. However, if you face a Katthveli (Cat Whale), you may save yourself by making loud noises, or tossing items overboard to distract it.
There are also some substances known to deter a Taumafiskurs (Bridled Whale), like juniper and cod live oil, that will probably work on other illhveli as well.
A few of the good whales, those that still cruise the seas, are strong enough enough and big enough to face a illhveli. The Blue Whale, the mightiest of the good whales, is one of them and has been recognized in poems and sagas as an in valuable ally of humans in their battles against the illhveli. A Blue Whale would intercept and fight any illhveli nearby a vessel or other smaller whales, protecting them. For that reason, harm or impeding Blue Whales is taboo, and killing one of those gentle giants may place a terrible curse on you.
In ancient times, the illhveli traveled the seas alongside other mythical creatures like the Hafgua and the Kraken, which were only slightly bigger and more terrifying than the evil whales themselves. And even if nowadays such monsters don't bother coming near the surface to prey on humans, it won't hurt your chances to get home unharmed after a long cruise in the Northern Seas to steer away of any suspicious-looking island, avoid saying the name of any illhveli out loud, and if you can find one, travel in a boat that is particularly fast or under the authority of captain with good knowledge of Icelandic sorcery and dark arts.