Nidhogg is a dragon that lives in Niffleheim. He consumes the souls of those who have fallen in battle.
Marvel Comics
Balder once traveled to Hel and along the way encountered Nidhogg. He then saw what happened to the souls belonging to those he had killed in battle. Nidhogg was also visited by the Lady Sif, while on her journey to learn the location of the cave of Aerndis.
Death
During the War of the Realms, Nidhogg attends a council of Niffleheim called by Muspelheim’s ruler Queen Sindr as her siege of the Land of the Dead begins. Sindr invites Nidhogg, the Disir and the Draugr to join her and bend the knee to their new Queen. Nidhogg refuses and engages in combat with Sindr, however he is slaughtered by a blade of Muspelheim as the first major casuality of the Siege of Niffleheim.