The Lyngbakur is the largest of all the illhveli, and indeed one of the largest creatures in the sea. In Icelandic lore only the Hafgua is bigger than the Lyngbakur.
However, despite its enormous size, the lyngbakur is rarely seen, and it does not go out searching for ships to sink the way the smaller illhveli do. Most often, the single part of this colossal beast that can be seen over the surface of the water is its back, a large hum like an island covered with a growth of heather. Its eyes, dorsally located, give the impression of circular pools of water. From a distance, the lyngbakur surface seems to be jet-black, but on closer inspection it reveals a myriad of mossy grey tones. Like all whales, it was a long tail and pectoral fins. All of them long and too thin for the rest of its body.
The lyngbakur is a slow swimmer, stopping midway to doze near the surface, disguised from the human as a heather-covered island. As mentioned in several Viking stories, it's possible to go right up and land on it without knowing one is walking on a living being, until the whale eventually awakes and dives, and anyone still on it drowns.
In southern Iceland, a folktale talks of a group of fishermen who stayed two days on the whale’s back before it sunk. They only escaped because their little boat was close and could boarded it before the whale submerged. One thing that will certainly wake a sleeping lyngbakur is trying to draw water from the pools that are its eyes. o
The lyngbakur feeds only once every three years, but when it does it engulfs anything in its path; fish, birds, whales and boats alike.
The Viking saga of Arrow-Odd relates includes an encounter with a lyngbakur sent by his Arrow-Odd's worst enemy: Ogmund. When Arrow-Odd stopped by a large heather-covered island, he had five of his crew disembark to find drinking water. Before long the island began to move, going underwater and drowning the unfortunate crewmen.
Saint Brendan (i.e., An early Irish monastic saint also referred to as "the Navigator") moored at a small island covered with sparse vegetation and with no sand on its shores. He and his followers spent the night praying on the island, but left next morning in a hurry as the ground began to shake. They returned to their ship in time, where they found out that they had been on the back of a Jasconius or Iascanus, a great whale that seems to be none other than the lyngbakur. The next time they encountered the whale, Saint Brendan fearlessly sang Easter Mass on it, and none were harmed.
It is said that there is only one lyngbakur still alive, and everyone that knows about these things, knows this last one will live until Armageddon. But not everyone agrees, however, if the Lybgbakur will die when the Armageddon comes, or if the Armageddon will come when the Lyngbakur dies.
-Davidsson, O. 1900. The Folk-lore of Icelandic Fishes. The Scottish Review.
-Hlidberg, J. B. and Aegisson, S.; McQueen, F. J. M. & Kjartansson, R., trans. 2011. Meeting with Monsters. JPV utgafa, Reykjavik.