What is "naglfari"

In Norse mythology, Naglfari is the father of Auðr by the personified night, Nótt. Naglfar is attested in a single mention in the Prose Edda (written in the 13th century by Snorri Sturluson) book Gylfaginning, where he is described as one of a series of three husbands of Nótt, and that the couple produced a son, Auðr. No additional information is provided about Naglfari.

Rudolf Simek theorizes that Snorri invented Naglfari but states that his reason for doing so is unknown.