In Norse mythology, Amsvartnir (Old Norse "pitch black") is a lake that is primarily known for its association with the island Lyngvi, where the gods bound the wolf Fenrir. This lake is solely mentioned in the Prose Edda book Gylfaginning, which was written in the 13th century by Snorri Sturluson.
In Gylfaginning, the figure of High, who is seated on a throne, informs Gangleri (a disguised King Gylfi) that the gods and Fenrir traveled across Amsvartnir to reach the island Lyngvi, where they eventually bound Fenrir.
It's important to note that Amsvartnir is not mentioned in any other surviving Norse mythological sources, and its sole mention is in Gylfaginning. Due to the lack of attestation in other sources, some scholars, like Rudolf Simek, suggest that Snorri Sturluson may have invented the lake Amsvartnir as part of his literary composition rather than drawing from earlier Norse mythology. This hypothesis underscores the complex nature of Norse mythology and the challenges of interpreting it, as some elements may have been elaborated or invented by later writers like Snorri.