Lyngvi is the island where the gods imprisoned Fenris Wolf.

It is described as looking like the crater of an extinct volcano in the sea and being covered in moss, which helps keep Fenris Wolf imprisoned.

Magnus Chase and the Gods of Asgard

The Sword of Summer

Magnus Chase, Samirah al-Abbas, Hearthstone and Blitzen arrive on the island and check on Fenris Wolf. They are found by Gunilla, two other Valkyries and Magnus’s hall mates. The son of Frey’s hall mates join him and fight Surt when his forces arrive. The wolf is cut free and battled by Sam until he is rebounded by Jack and Magnus banishes Surt and his forces. With the Valkyries dead the remaining forces return to the Hotel Valhalla.

Locations (CHB)
Magical Locations: Aeolia | Camp Half-Blood | Camp Jupiter | Cave of Trophonius | C.C.'s Spa and Resort | Daedalus' Workshop | Lotus Hotel and Casino | Mount Othrys | Ogygia | Olympus | Pan's Cave | Sea of Monsters | The Labyrinth | Tartarus | Underworld | Waystation
Cities, States, and Towns: Alaska | Bar Harbor | Bologna | Chicago | Detroit | Gila Claw | Indianapolis | Jamestown | Montauk | New Mexico | New York City | Palm Springs | Quebec | San Francisco | Dalmatia | Vancouver | Venice | Westport
Other Locations: Apennine Mountains | Aunty Em's Gnome Emporium | Carlsbad Caverns | Crusty's Water Bed Palace | Empire State Building | Gateway Arch | Grand Canyon | Greece | Hoover Dam | Italy | Junkyard of the Gods | Mount Diablo | Mount Etna | Mount Saint Helens | Mount Tamalpais | Pikes Peak | Polyphemus' Island | ROFL | Rome | Triple G Ranch | U.S.A. | Waterland