Art by A Book of Creatures
The Hrökkáll is an eel with iron-hard scales and razor sharp fins. It is 2 feet in length and lives in stagnant ponds, polluted waters, and occasionally rivers. It secretes a corrosive poison that it can utilize to burrow through the earth. They lie in wait for someone to step into the waters in which they live, then coil around their legs. They will then proceed to slice through flesh and bone. Sheep are said to be safe from the Hrökkáll because their legs are too narrow for them to gain a hold.
Today, Hrökkáll is the Icelandic word for the Electrophorus electricus, a species of electric eel.
According to folklore, there was once a wizard who gave life to a dead half-rotten eel, which became an evil and toxic being. The resurrected eel was transformed into the first beast of its kind, and its descendants went on to live in polluted waters.