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Ámsvartnir (Old Norse = 'red-black one' or 'completely black one') is the name of a lake invented by SnorriSturluson. The island of Lyngvi, on which the Fenrisúlfr is chained up until the Ragnarǫk, lies in this lake. [1][2]
References
↑Rudolf Simek: Dictionary of Northern Mythology (1993)
↑Leiv Heggstad, Finn Hødnebø og Erik Simensen: Norrøn Ordbok (1997)