Nidhogg is a dragon that lives in Niffleheim. He consumes the souls of those who have fallen in battle.
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.
Recently Nidhogg was visited by the Lady Sif, while on her journey to learn the location of the cave of Aerndis.